"onCompleteDone" is executed asynchronously.

autoload/ddc.vim

@@ -53,7 +53,7 @@ function ddc#on_complete_done(completed_item) abort
     return
   endif
 
-  call ddc#denops#_request('onCompleteDone', [a:completed_item])
+  call ddc#denops#_notify('onCompleteDone', [a:completed_item])
 endfunction
 
 function ddc#syntax_in(groups) abort

doc/ddc.txt

@@ -1007,6 +1007,9 @@ getPreviewer		(function)			(Optional)
 onCompleteDone		(function)			(Optional)
 		Called after the completion text is inserted.
 		It is useful to substitute text after completion.
+		NOTE: It is executed asynchronously to optimize the
+		performance.  If you need to execute it synchronously, you
+		should use |CompleteDone| autocmd instead.
 		NOTE: |ddc-item-attribute-user_data| must be "UserData" to use
 		the feature.
 		NOTE: If it changes the text, it should call
@@ -1684,6 +1687,9 @@ https://github.com/Shougo/ddc-ui-pum
 ==============================================================================
 COMPATIBILITY                                              *ddc-compatibility*
 
+2024.08.16
+* "onCompleteDone" is executed asynchronously.
+
 2023.12.06
 * Change "ddc#set_static_import_path()" spec.