diff --git a/packages/interactivity/CHANGELOG.md b/packages/interactivity/CHANGELOG.md
index 5092fe4feb807..904892788ef7c 100644
--- a/packages/interactivity/CHANGELOG.md
+++ b/packages/interactivity/CHANGELOG.md
@@ -4,9 +4,21 @@
 
 ### Bug Fixes
 
+-   Fix reactivity of undefined objects and arrays added with `deepMerge()` ([#66183](https://github.com/WordPress/gutenberg/pull/66183)).
+
+## 6.10.0 (2024-10-16)
+
+### Internal
+
+-   Upgrade preact libraries [#66008](https://github.com/WordPress/gutenberg/pull/66008).
+
+### Bug Fixes
+
 -   Fix an issue where "default" could not be used as a directive suffix ([#65815](https://github.com/WordPress/gutenberg/pull/65815)).
 -   Correctly handle lazily added, deeply nested properties with `deepMerge()` ([#65465](https://github.com/WordPress/gutenberg/pull/65465)).
 
+## 6.9.0 (2024-10-03)
+
 ## 6.8.0 (2024-09-19)
 
 ### Enhancements
diff --git a/packages/interactivity/src/proxies/state.ts b/packages/interactivity/src/proxies/state.ts
index f63c84738bcb1..d4d573acba884 100644
--- a/packages/interactivity/src/proxies/state.ts
+++ b/packages/interactivity/src/proxies/state.ts
@@ -335,7 +335,10 @@ const deepMergeRecursive = (
 			if ( isNew || ( override && ! isPlainObject( target[ key ] ) ) ) {
 				target[ key ] = {};
 				if ( propSignal ) {
-					propSignal.setValue( target[ key ] );
+					const ns = getNamespaceFromProxy( proxy );
+					propSignal.setValue(
+						proxifyState( ns, target[ key ] as Object )
+					);
 				}
 			}
 			if ( isPlainObject( target[ key ] ) ) {
@@ -344,7 +347,11 @@ const deepMergeRecursive = (
 		} else if ( override || isNew ) {
 			Object.defineProperty( target, key, desc );
 			if ( propSignal ) {
-				propSignal.setValue( desc.value );
+				const { value } = desc;
+				const ns = getNamespaceFromProxy( proxy );
+				propSignal.setValue(
+					shouldProxy( value ) ? proxifyState( ns, value ) : value
+				);
 			}
 		}
 	}
diff --git a/packages/interactivity/src/proxies/test/deep-merge.ts b/packages/interactivity/src/proxies/test/deep-merge.ts
index f475385a43787..267e4850f9af9 100644
--- a/packages/interactivity/src/proxies/test/deep-merge.ts
+++ b/packages/interactivity/src/proxies/test/deep-merge.ts
@@ -379,7 +379,10 @@ describe( 'Interactivity API', () => {
 			const target = proxifyState< any >( 'test', { a: 1, b: 2 } );
 			const source = { a: 1, b: 2, c: 3 };
 
-			const spy = jest.fn( () => Object.keys( target ) );
+			let keys: any;
+			const spy = jest.fn( () => {
+				keys = Object.keys( target );
+			} );
 			effect( spy );
 
 			expect( spy ).toHaveBeenCalledTimes( 1 );
@@ -387,7 +390,7 @@ describe( 'Interactivity API', () => {
 			deepMerge( target, source, false );
 
 			expect( spy ).toHaveBeenCalledTimes( 2 );
-			expect( spy ).toHaveLastReturnedWith( [ 'a', 'b', 'c' ] );
+			expect( keys ).toEqual( [ 'a', 'b', 'c' ] );
 		} );
 
 		it( 'should handle deeply nested properties that are initially undefined', () => {
@@ -412,6 +415,13 @@ describe( 'Interactivity API', () => {
 
 			// Reading the value directly should also work
 			expect( target.a.b.c.d ).toBe( 'test value' );
+
+			// Modify the nested value
+			target.a.b.c.d = 'new test value';
+
+			// The effect should be called again
+			expect( spy ).toHaveBeenCalledTimes( 3 );
+			expect( deepValue ).toBe( 'new test value' );
 		} );
 
 		it( 'should overwrite values that become objects', () => {
@@ -462,5 +472,33 @@ describe( 'Interactivity API', () => {
 			expect( target.message.content ).toBeUndefined();
 			expect( target.message.fontStyle ).toBeUndefined();
 		} );
+
+		it( 'should keep reactivity of arrays that are initially undefined', () => {
+			const target: any = proxifyState( 'test', {} );
+
+			let deepValue: any;
+			const spy = jest.fn( () => {
+				deepValue = target.array?.[ 0 ];
+			} );
+			effect( spy );
+
+			// Initial call, the deep value is undefined
+			expect( spy ).toHaveBeenCalledTimes( 1 );
+			expect( deepValue ).toBeUndefined();
+
+			// Use deepMerge to add an array to the target
+			deepMerge( target, { array: [ 'value 1' ] } );
+
+			// The effect should be called again
+			expect( spy ).toHaveBeenCalledTimes( 2 );
+			expect( deepValue ).toBe( 'value 1' );
+
+			// Modify the array value
+			target.array[ 0 ] = 'value 2';
+
+			// The effect should be called again
+			expect( spy ).toHaveBeenCalledTimes( 3 );
+			expect( deepValue ).toBe( 'value 2' );
+		} );
 	} );
 } );