When a room is deleted through start_shutdown_and_purge_room (e.g. using the admin API), the users in the room don't receive a leave event or any other signal that it should be removed from the room list.

This leaves clients with stuck rooms that cannot have messages sent into. When that happens, the issue can only be fixed manually by the user: the user has to 'Clear cache and reload' (or equivalent).

Noticed as at Synapse v1.88.0 (though I have heard this issue mentioned earlier too; older Synapse-dev discussion)