[Core data]: Fix wrong store results when page receives less items th…

…at what is stored

packages/core-data/src/queried-data/reducer.js

@@ -56,10 +56,10 @@ export function getMergedItemIds( itemIds, nextItemIds, page, perPage ) {
 
 	for ( let i = 0; i < size; i++ ) {
 		// Preserve existing item ID except for subset of range of next items.
+		// We need to check against the possible maximum upper boundary because
+		// a page could recieve less items than what was previously stored.
 		const isInNextItemsRange =
-			i >= nextItemIdsStartIndex &&
-			i < nextItemIdsStartIndex + nextItemIds.length;
-
+			i >= nextItemIdsStartIndex && i < nextItemIdsStartIndex + perPage;
 		mergedItemIds[ i ] = isInNextItemsRange
 			? nextItemIds[ i - nextItemIdsStartIndex ]
 			: itemIds?.[ i ];

packages/core-data/src/queried-data/selectors.js

@@ -55,7 +55,7 @@ function getQueriedItemsUncached( state, query ) {
 
 		// Having a target item ID doesn't guarantee that this object has been queried.
 		if ( ! state.items[ context ]?.hasOwnProperty( itemId ) ) {
-			return null;
+			continue;
 		}
 
 		const item = state.items[ context ][ itemId ];

packages/core-data/src/queried-data/test/reducer.js

@@ -64,6 +64,17 @@ describe( 'getMergedItemIds', () => {
 
 		expect( result ).toEqual( [ 1, 3, 4 ] );
 	} );
+	it( 'should update a page properly if less items are provided than previously stored', () => {
+		let original = deepFreeze( [ 1, 2, 3 ] );
+		let result = getMergedItemIds( original, [ 1, 2 ], 1, 3 );
+
+		expect( result ).toEqual( [ 1, 2 ] );
+
+		original = deepFreeze( [ 1, 2, 3, 4, 5, 6 ] );
+		result = getMergedItemIds( original, [ 9 ], 2, 2 );
+
+		expect( result ).toEqual( [ 1, 2, 9, undefined, 5, 6 ] );
+	} );
 } );
 
 describe( 'itemIsComplete', () => {