diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 9490c6c0d7..bdc0d28819 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -3507,6 +3507,8 @@ class SyncHandler:
                 knock = await self.store.get_event(event.event_id)
                 knocked.append(KnockedSyncResult(room_id=event.room_id, knock=knock))
             elif event.membership in (Membership.LEAVE, Membership.BAN):
+                # Former members of an announcement room get the synthetic join
+                # instead of this leave or ban.
                 if event.room_id in sync_result_builder.announcement_room_ids:
                     continue