Only check membership events persisted after the get rooms call

synapse/handlers/sync.py

@@ -1324,6 +1324,7 @@ async def generate_sync_result(
 
         # Note: we get the users room list *before* we get the current token, this
         # avoids checking back in history if rooms are joined after the token is fetched.
+        token_before_rooms = self.event_sources.get_current_token()
         mutable_joined_room_ids = set(await self.store.get_rooms_for_user(user_id))
 
         # NB: The now_token gets changed by some of the generate_sync_* methods,
@@ -1352,6 +1353,13 @@ async def generate_sync_result(
             # latest change is JOIN.
 
             for room_id, event in mem_last_change_by_room_id.items():
+                assert event.internal_metadata.stream_ordering
+                if (
+                    event.internal_metadata.stream_ordering
+                    < token_before_rooms.room_key.stream
+                ):
+                    continue
+
                 logger.info(
                     "User membership change between getting rooms and current token: %s %s %s",
                     user_id,