rejoin logic was throwing away the prev_batch token

src/matrix/room/timeline/persistence/SyncWriter.js

@@ -200,10 +200,10 @@ export class SyncWriter {
                 const index = events.findIndex(event => event.event_id === lastEventId);
                 if (index !== -1) {
                     log.set("overlap_event_id", lastEventId);
-                    return {
+                    return Object.assign({}, timeline, {
                         limited: false,
-                        events: events.slice(index + 1)
-                    };
+                        events: events.slice(index + 1),
+                    });
                 }
             }
         }