do onLeft (which clears the LL members) as late as possible

to avoid chance that something might call loadMembersIfNeeded
on the room and load them back again.

src/sync.js

@@ -1106,7 +1106,6 @@ SyncApi.prototype._processSyncResponse = async function(
     leaveRooms.forEach(function(leaveObj) {
         const room = leaveObj.room;
         room.setSyncedMembership("leave");
-        room.onLeft();
 
         const stateEvents =
             self._mapSyncEventsFormat(leaveObj.state, room);
@@ -1135,6 +1134,8 @@ SyncApi.prototype._processSyncResponse = async function(
         accountDataEvents.forEach(function(e) {
             client.emit("event", e);
         });
+
+        room.onLeft();
     });
 
     // update the notification timeline, if appropriate.