diff --git a/src/dtx/dtx_coll.c b/src/dtx/dtx_coll.c
index 863307e9a7f..7e7c8199155 100644
--- a/src/dtx/dtx_coll.c
+++ b/src/dtx/dtx_coll.c
@@ -80,8 +80,12 @@ dtx_coll_prep_ult(void *arg)
 				DP_UUID(cont->sc_uuid), DP_RC(rc));
 	}
 
-	if (dcpa->dcpa_result != 0)
+	if (dcpa->dcpa_result != 0) {
+		if (dcpa->dcpa_result != -DER_INPROGRESS && dcpa->dcpa_result != -DER_NONEXIST)
+			D_ERROR("Failed to load mbs for "DF_DTI", opc %u: "DF_RC"\n",
+				DP_DTI(&dci->dci_xid), opc, DP_RC(rc));
 		goto out;
+	}
 
 	dcpa->dcpa_result = dtx_coll_prep(dci->dci_po_uuid, dcpa->dcpa_oid, &dci->dci_xid, mbs, -1,
 					  dci->dci_version, cont->sc_pool->spc_map_version,
diff --git a/src/dtx/dtx_rpc.c b/src/dtx/dtx_rpc.c
index e654047a621..b530b92fe61 100644
--- a/src/dtx/dtx_rpc.c
+++ b/src/dtx/dtx_rpc.c
@@ -976,8 +976,11 @@ dtx_refresh_internal(struct ds_cont_child *cont, int *check_count, d_list_t *che
 		if (dsp->dsp_mbs == NULL) {
 			rc = vos_dtx_load_mbs(cont->sc_hdl, &dsp->dsp_xid, NULL, &dsp->dsp_mbs);
 			if (rc != 0) {
-				if (rc < 0 && rc != -DER_NONEXIST && for_io)
+				if (rc < 0 && rc != -DER_NONEXIST && for_io) {
+					D_ERROR("Failed to load mbs for "DF_DTI": "DF_RC"\n",
+						DP_DTI(&dsp->dsp_xid), DP_RC(rc));
 					goto out;
+				}
 
 				drop = true;
 				goto next;
diff --git a/src/object/cli_coll.c b/src/object/cli_coll.c
index e05abadf3cf..d2d4a0d4336 100644
--- a/src/object/cli_coll.c
+++ b/src/object/cli_coll.c
@@ -178,6 +178,10 @@ obj_coll_oper_args_init(struct coll_oper_args *coa, struct dc_object *obj, bool
 		if (coa->coa_sparse == 0)
 			coa->coa_dct_cap = obj_ranks;
 	}
+
+	/* Temporarily save obj->cob_min_rank for verification during obj_coll_prep_one. */
+	coa->coa_min_rank = obj->cob_min_rank;
+
 	D_RWLOCK_UNLOCK(&obj->cob_lock);
 
 	if (coa->coa_sparse) {
@@ -208,7 +212,6 @@ obj_coll_oper_args_init(struct coll_oper_args *coa, struct dc_object *obj, bool
 		coa->coa_dct_nr = -1;
 	}
 
-	coa->coa_max_dct_sz = 0;
 	coa->coa_max_shard_nr = 0;
 	coa->coa_max_bitmap_sz = 0;
 	coa->coa_target_nr = 0;
@@ -236,17 +239,55 @@ obj_coll_oper_args_fini(struct coll_oper_args *coa)
 	coa->coa_dct_nr = 0;
 }
 
+static void
+obj_coll_collapse_one(struct coll_oper_args *coa, struct daos_coll_target *dct,
+		      uint32_t *size, bool copy)
+{
+	struct daos_coll_shard	*dcs;
+	uint32_t		 dct_size;
+	int			 i;
+
+	/* The size may be over estimated, no matter. */
+	dct_size = sizeof(*dct) + dct->dct_bitmap_sz +
+		   sizeof(dct->dct_shards[0]) * (dct->dct_max_shard + 1);
+
+	for (i = 0; i <= dct->dct_max_shard; i++) {
+		dcs = &dct->dct_shards[i];
+		if (dcs->dcs_nr > 1)
+			dct_size += sizeof(dcs->dcs_buf[0]) * dcs->dcs_nr;
+	}
+
+	if (coa->coa_for_modify)
+		dct_size += sizeof(dct->dct_tgt_ids[0]) * dct->dct_tgt_nr;
+
+	if (coa->coa_max_dct_sz < dct_size)
+		coa->coa_max_dct_sz = dct_size;
+
+	if (copy)
+		memcpy(&coa->coa_dcts[coa->coa_dct_nr], dct, sizeof(*dct));
+
+	coa->coa_dct_nr++;
+	*size += dct_size;
+}
+
+struct obj_coll_tree_args {
+	struct coll_oper_args	*coa;
+	uint32_t		*size;
+};
+
 static int
 obj_coll_tree_cb(daos_handle_t ih, d_iov_t *key, d_iov_t *val, void *arg)
 {
-	struct coll_oper_args	*coa = arg;
-	struct daos_coll_target	*dct = val->iov_buf;
+	struct obj_coll_tree_args	*octa = arg;
+	struct coll_oper_args		*coa = octa->coa;
+	struct daos_coll_target		*dct = val->iov_buf;
 
 	D_ASSERTF(coa->coa_dct_nr < coa->coa_dct_cap,
 		  "Too short pre-allcoated dct_array: %u vs %u\n",
 		  coa->coa_dct_nr, coa->coa_dct_cap);
+	D_ASSERT(dct->dct_bitmap != NULL);
 
-	memcpy(&coa->coa_dcts[coa->coa_dct_nr++], dct, sizeof(*dct));
+	obj_coll_collapse_one(coa, dct, octa->size, true);
 
 	/* The following members have been migrated into coa->coa_dcts. */
 	dct->dct_bitmap = NULL;
@@ -259,6 +300,7 @@ obj_coll_tree_cb(daos_handle_t ih, d_iov_t *key, d_iov_t *val, void *arg)
 static int
 obj_coll_collapse_tree(struct coll_oper_args *coa, uint32_t *size)
 {
+	struct obj_coll_tree_args	 octa;
 	struct coll_sparse_targets	*tree = coa->coa_tree;
 	int				 rc = 0;
 
@@ -270,7 +312,14 @@ obj_coll_collapse_tree(struct coll_oper_args *coa, uint32_t *size)
 		D_GOTO(out, rc = -DER_NOMEM);
 
 	coa->coa_sparse = 0;
-	rc = dbtree_iterate(tree->cst_tree_hdl, DAOS_INTENT_DEFAULT, false, obj_coll_tree_cb, coa);
+	coa->coa_raw_sparse = 1;
+	coa->coa_dct_nr = 0;
+	coa->coa_max_dct_sz = 0;
+
+	octa.coa = coa;
+	octa.size = size;
+	rc = dbtree_iterate(tree->cst_tree_hdl, DAOS_INTENT_DEFAULT, false,
+			    obj_coll_tree_cb, &octa);
 	if (rc == 0)
 		D_ASSERTF(coa->coa_dct_nr == coa->coa_dct_cap,
 			  "Something is wrong when prepare coll target array: %u vs %u\n",
@@ -287,35 +336,19 @@ static int
 obj_coll_collapse_array(struct coll_oper_args *coa, uint32_t *size)
 {
 	struct daos_coll_target	*dct;
-	struct daos_coll_shard	*dcs;
-	uint32_t		 dct_size;
 	int			 i;
-	int			 j;
 
-	for (i = 0, *size = 0, coa->coa_dct_nr = 0; i < coa->coa_dct_cap; i++) {
+	for (i = 0, *size = 0, coa->coa_dct_nr = 0, coa->coa_max_dct_sz = 0;
+	     i < coa->coa_dct_cap; i++) {
 		dct = &coa->coa_dcts[i];
 		if (dct->dct_bitmap != NULL) {
-			/* The size may be over estimated, no matter. */
-			dct_size = sizeof(*dct) + dct->dct_bitmap_sz +
-				   sizeof(dct->dct_shards[0]) * (dct->dct_max_shard + 1);
-
-			for (j = 0; j <= dct->dct_max_shard; j++) {
-				dcs = &dct->dct_shards[j];
-				if (dcs->dcs_nr > 1)
-					dct_size += sizeof(dcs->dcs_buf[0]) * dcs->dcs_nr;
-			}
-
-			if (coa->coa_for_modify)
-				dct_size += sizeof(dct->dct_tgt_ids[0]) * dct->dct_tgt_nr;
-
-			if (coa->coa_max_dct_sz < dct_size)
-				coa->coa_max_dct_sz = dct_size;
-
-			if (coa->coa_dct_nr < i)
-				memcpy(&coa->coa_dcts[coa->coa_dct_nr], dct, sizeof(*dct));
-
-			coa->coa_dct_nr++;
-			*size += dct_size;
+			if (coa->coa_dct_nr < i && coa->coa_dct_nr > 0)
+				D_ASSERTF(coa->coa_dcts[coa->coa_dct_nr - 1].dct_rank <
+					  dct->dct_rank,
+					  "Unsorted ranks in target array %u vs %u\n",
+					  coa->coa_dcts[coa->coa_dct_nr - 1].dct_rank,
+					  dct->dct_rank);
+			obj_coll_collapse_one(coa, dct, size, coa->coa_dct_nr < i);
 		}
 	}
 
@@ -373,8 +406,9 @@ obj_coll_prep_one(struct coll_oper_args *coa, struct dc_object *obj,
 
 	D_RWLOCK_RDLOCK(&obj->cob_lock);
 
-	D_ASSERTF(shard->do_target_rank <= obj->cob_max_rank,
-		  "Unexpected shard with rank %u > %u\n", shard->do_target_rank, obj->cob_max_rank);
+	D_ASSERTF(coa->coa_min_rank == obj->cob_min_rank,
+		  "Object "DF_OID" layout has been changed unexpectedly %u => %u, idx %u, ver %u\n",
+		  DP_OID(obj->cob_md.omd_id), coa->coa_min_rank, obj->cob_min_rank, idx, map_ver);
 	D_ASSERTF(shard->do_target_rank >= obj->cob_min_rank,
 		  "Unexpected shard with rank %u < %u\n", shard->do_target_rank, obj->cob_min_rank);
 
@@ -669,7 +703,6 @@ dc_obj_coll_punch(tse_task_t *task, struct dc_object *obj, struct dtx_epoch *epo
 	uint32_t			 tgt_size = 0;
 	uint32_t			 mbs_max_size;
 	uint32_t			 inline_size;
-	uint32_t			 flags = ORF_LEADER;
 	uint32_t			 leader = -1;
 	uint32_t			 len;
 	int				 rc;
@@ -746,6 +779,12 @@ dc_obj_coll_punch(tse_task_t *task, struct dc_object *obj, struct dtx_epoch *epo
 		memcpy(dct, &tmp_tgt, sizeof(tmp_tgt));
 	}
 
+	/* 'shard' is on the leader target that is must be the coa->coa_dcts[0]. */
+	D_ASSERTF(shard->do_target_rank == coa->coa_dcts[0].dct_rank,
+		  "Object "DF_OID" target array corrupted: rank %u vs %ur, nr %u\n",
+		  DP_OID(obj->cob_md.omd_id), shard->do_target_rank,
+		  coa->coa_dcts[0].dct_rank, coa->coa_dct_nr);
+
 	rc = dc_obj_coll_punch_mbs(coa, obj, shard->do_target_id, &mbs);
 	if (rc < 0)
 		goto out;
@@ -767,12 +806,14 @@ dc_obj_coll_punch(tse_task_t *task, struct dc_object *obj, struct dtx_epoch *epo
 	if (rc != 0)
 		goto out;
 
+	auxi->flags = ORF_LEADER;
 	if (auxi->io_retry) {
-		flags |= ORF_RESEND;
+		auxi->flags |= ORF_RESEND;
 		/* Reset @enqueue_id if resend to new leader. */
 		if (spa->pa_auxi.target != shard->do_target_id)
 			spa->pa_auxi.enqueue_id = 0;
 	} else {
+		auxi->flags &= ~ORF_RESEND;
 		spa->pa_auxi.obj_auxi = auxi;
 		daos_dti_gen(&spa->pa_dti, false);
 	}
@@ -781,14 +822,15 @@ dc_obj_coll_punch(tse_task_t *task, struct dc_object *obj, struct dtx_epoch *epo
 	spa->pa_auxi.shard = shard->do_shard_idx;
 
 	if (obj_is_ec(obj))
-		flags |= ORF_EC;
+		auxi->flags |= ORF_EC;
 
 	mbs_max_size = sizeof(*mbs) + mbs->dm_data_size +
 		       sizeof(coa->coa_targets[0]) * coa->coa_max_shard_nr + coa->coa_max_bitmap_sz;
 
 	return dc_obj_shard_coll_punch(shard, spa, mbs, mbs_max_size, cpca.cpca_bulks, tgt_size,
 				       coa->coa_dcts, coa->coa_dct_nr, coa->coa_max_dct_sz, epoch,
-				       args->flags, flags, map_ver, &auxi->map_ver_reply, task);
+				       args->flags, auxi->flags, map_ver,
+				       &auxi->map_ver_reply, task);
 
 out:
 	if (rc > 0)
diff --git a/src/object/cli_shard.c b/src/object/cli_shard.c
index 36c5c5f1e0c..0c9dfc1418e 100644
--- a/src/object/cli_shard.c
+++ b/src/object/cli_shard.c
@@ -1453,9 +1453,10 @@ obj_shard_coll_punch_cb(tse_task_t *task, void *data)
 
 	DL_CDEBUG(task->dt_result < 0, DLOG_ERR, DB_IO, task->dt_result,
 		  "DAOS_OBJ_RPC_COLL_PUNCH RPC %p for "DF_UOID" with DTX "
-		  DF_DTI" for task %p, map_ver %u/%u, flags %lx/%x", rpc, DP_UOID(ocpi->ocpi_oid),
-		  DP_DTI(&ocpi->ocpi_xid), task, ocpi->ocpi_map_ver, *cb_args->cpca_ver,
-		  (unsigned long)ocpi->ocpi_api_flags, ocpi->ocpi_flags);
+		  DF_DTI" for task %p, map_ver %u/%u, flags %lx/%x, %s layout",
+		  rpc, DP_UOID(ocpi->ocpi_oid), DP_DTI(&ocpi->ocpi_xid), task, ocpi->ocpi_map_ver,
+		  *cb_args->cpca_ver, (unsigned long)ocpi->ocpi_api_flags, ocpi->ocpi_flags,
+		  cb_args->cpca_shard_args->pa_coa.coa_raw_sparse ? "sparse" : "continuous");
 
 	crt_req_decref(rpc);
 
diff --git a/src/object/obj_internal.h b/src/object/obj_internal.h
index ec6aa3b817e..c3a9a48c970 100644
--- a/src/object/obj_internal.h
+++ b/src/object/obj_internal.h
@@ -292,10 +292,15 @@ struct coll_oper_args {
 	struct shard_auxi_args	 coa_auxi;
 	int			 coa_dct_nr;
 	uint32_t		 coa_dct_cap;
-	uint32_t		 coa_max_dct_sz;
+	union {
+		uint32_t	 coa_max_dct_sz;
+		/* Temporarily save obj->cob_min_rank for verification during obj_coll_prep_one. */
+		uint32_t	 coa_min_rank;
+	};
 	uint8_t			 coa_max_shard_nr;
 	uint8_t			 coa_max_bitmap_sz;
 	uint8_t			 coa_for_modify:1,
+				 coa_raw_sparse:1,
 				 coa_sparse:1;
 	uint8_t			 coa_target_nr;
 	/*
diff --git a/src/object/obj_utils.c b/src/object/obj_utils.c
index 7bf0ef4aaf9..82d91c966ac 100644
--- a/src/object/obj_utils.c
+++ b/src/object/obj_utils.c
@@ -633,7 +633,7 @@ obj_coll_disp_dest(struct obj_coll_disp_cursor *ocdc, struct daos_coll_target *t
 		 * use the "cur_pos" as the relay engine.
 		 */
 		pos = rand % (ocdc->tgt_nr - ocdc->cur_pos) + ocdc->cur_pos;
-		if (pos != ocdc->cur_pos && tgts[pos].dct_rank > dct->dct_rank) {
+		if (pos > ocdc->cur_pos && tgts[pos].dct_rank > dct->dct_rank) {
 			memcpy(&tmp, &tgts[pos], sizeof(tmp));
 			memcpy(&tgts[pos], dct, sizeof(tmp));
 			memcpy(dct, &tmp, sizeof(tmp));
diff --git a/src/object/srv_coll.c b/src/object/srv_coll.c
index 2a152b47bd6..d1c529a1c62 100644
--- a/src/object/srv_coll.c
+++ b/src/object/srv_coll.c
@@ -239,9 +239,15 @@ obj_coll_punch_prep(struct obj_coll_punch_in *ocpi, struct daos_coll_target *dct
 	int			 i;
 	int			 j;
 
-	/* dcts[0] is for current engine. */
-	if (dcts[0].dct_bitmap == NULL || dcts[0].dct_bitmap_sz == 0 ||
-	    dcts[0].dct_shards == NULL) {
+	/* dcts[0] must be for current engine. */
+	if (unlikely(dcts[0].dct_rank != dss_self_rank())) {
+		D_ERROR("Invalid targets array: rank %u vs %u, nr %u, flags %x\n",
+			dcts[0].dct_rank, dss_self_rank(), dct_nr, ocpi->ocpi_flags);
+		D_GOTO(out, rc = -DER_INVAL);
+	}
+
+	if (unlikely(dcts[0].dct_bitmap == NULL || dcts[0].dct_bitmap_sz == 0 ||
+		     dcts[0].dct_shards == NULL)) {
 		D_ERROR("Invalid input for current engine: bitmap %s, bitmap_sz %u, shards %s\n",
 			dcts[0].dct_bitmap == NULL ? "empty" : "non-empty", dcts[0].dct_bitmap_sz,
 			dcts[0].dct_shards == NULL ? "empty" : "non-empty");
@@ -302,6 +308,11 @@ obj_coll_punch_prep(struct obj_coll_punch_in *ocpi, struct daos_coll_target *dct
 
 	/* Set i = 1 to skip leader_rank. */
 	for (i = 1; i < dct_nr; i++) {
+		if (unlikely(dcts[0].dct_rank == dcts[i].dct_rank)) {
+			D_ERROR("Duplicated leader rank %u at %d\n", dcts[0].dct_rank, i);
+			D_GOTO(out, rc = -DER_INVAL);
+		}
+
 		dce->dce_ranks->rl_ranks[i - 1] = dcts[i].dct_rank;
 		if (max_rank < dcts[i].dct_rank)
 			max_rank = dcts[i].dct_rank;
diff --git a/src/object/srv_obj.c b/src/object/srv_obj.c
index a51682b4785..2f4d91f0020 100644
--- a/src/object/srv_obj.c
+++ b/src/object/srv_obj.c
@@ -5605,12 +5605,12 @@ ds_obj_coll_punch_handler(crt_rpc_t *rpc)
 
 	D_DEBUG(DB_IO, "(%s) handling collective punch RPC %p for obj "
 		DF_UOID" on XS %u/%u epc "DF_X64" pmv %u, with dti "
-		DF_DTI", forward width %u, forward depth %u\n",
+		DF_DTI", forward width %u, forward depth %u, flags %x\n",
 		(ocpi->ocpi_flags & ORF_LEADER) ? "leader" :
 		(ocpi->ocpi_tgts.ca_count == 1 ? "non-leader" : "relay-engine"),
 		rpc, DP_UOID(ocpi->ocpi_oid), dmi->dmi_xs_id, dmi->dmi_tgt_id,
 		ocpi->ocpi_epoch, ocpi->ocpi_map_ver, DP_DTI(&ocpi->ocpi_xid),
-		ocpi->ocpi_disp_width, ocpi->ocpi_disp_depth);
+		ocpi->ocpi_disp_width, ocpi->ocpi_disp_depth, ocpi->ocpi_flags);
 
 	D_ASSERT(dmi->dmi_xs_id != 0);
 
@@ -5747,13 +5747,13 @@ ds_obj_coll_punch_handler(crt_rpc_t *rpc)
 	DL_CDEBUG(rc != 0 && rc != -DER_INPROGRESS && rc != -DER_TX_RESTART, DLOG_ERR, DB_IO, rc,
 		  "(%s) handled collective punch RPC %p for obj "DF_UOID" on XS %u/%u epc "
 		  DF_X64" pmv %u/%u, with dti "DF_DTI", bulk_tgt_sz %u, bulk_tgt_nr %u, "
-		  "tgt_nr %u, forward width %u, forward depth %u",
+		  "tgt_nr %u, forward width %u, forward depth %u, flags %x",
 		  (ocpi->ocpi_flags & ORF_LEADER) ? "leader" :
 		  (ocpi->ocpi_tgts.ca_count == 1 ? "non-leader" : "relay-engine"), rpc,
 		  DP_UOID(ocpi->ocpi_oid), dmi->dmi_xs_id, dmi->dmi_tgt_id, ocpi->ocpi_epoch,
 		  ocpi->ocpi_map_ver, max_ver, DP_DTI(&ocpi->ocpi_xid), ocpi->ocpi_bulk_tgt_sz,
 		  ocpi->ocpi_bulk_tgt_nr, (unsigned int)ocpi->ocpi_tgts.ca_count,
-		  ocpi->ocpi_disp_width, ocpi->ocpi_disp_depth);
+		  ocpi->ocpi_disp_width, ocpi->ocpi_disp_depth, ocpi->ocpi_flags);
 
 	obj_punch_complete(rpc, rc, max_ver);
 
diff --git a/src/vos/vos_dtx.c b/src/vos/vos_dtx.c
index 1c60f781507..3600397ebff 100644
--- a/src/vos/vos_dtx.c
+++ b/src/vos/vos_dtx.c
@@ -1976,9 +1976,6 @@ vos_dtx_load_mbs(daos_handle_t coh, struct dtx_id *dti, daos_unit_oid_t *oid,
 			rc = -DER_INPROGRESS;
 	}
 
-	if (rc < 0)
-		D_ERROR("Failed to load mbs for "DF_DTI": "DF_RC"\n", DP_DTI(dti), DP_RC(rc));
-
 	return rc;
 }