diff --git a/backport-changelog/7.0/11716.md b/backport-changelog/7.0/11716.md
new file mode 100644
index 00000000000000..2d5be7c7f232e6
--- /dev/null
+++ b/backport-changelog/7.0/11716.md
@@ -0,0 +1,3 @@
+https://github.com/WordPress/wordpress-develop/pull/11716
+
+* https://github.com/WordPress/gutenberg/pull/77980
diff --git a/lib/compat/wordpress-7.0/class-wp-http-polling-sync-server.php b/lib/compat/wordpress-7.0/class-wp-http-polling-sync-server.php
index ac3680ebb32f24..2d1a530ca7d085 100644
--- a/lib/compat/wordpress-7.0/class-wp-http-polling-sync-server.php
+++ b/lib/compat/wordpress-7.0/class-wp-http-polling-sync-server.php
@@ -500,9 +500,14 @@ private function process_sync_update( string $room, int $client_id, int $cursor,
 						return $this->add_update( $room, $client_id, $type, $data );
 					}
 
-					// Reaching this point means there's a newer compaction, so we can
-					// silently ignore this one.
-					return true;
+					/*
+					 * A newer compaction already advanced the cursor, but we
+					 * can not safely drop an update. The incoming bytes still encode
+					 * operations other clients may not have seen, so store them as a
+					 * regular update. Y.applyUpdateV2 merges state-as-update blobs
+					 * idempotently, so overlap with the existing compaction is safe.
+					 */
+					return $this->add_update( $room, $client_id, self::UPDATE_TYPE_UPDATE, $data );
 
 				case self::UPDATE_TYPE_SYNC_STEP1:
 				case self::UPDATE_TYPE_SYNC_STEP2: