When a remote user leaves the last room shared with the local homeserver, their cached device list is not invalidated

[matrixbot]

This issue has been migrated from #13651.

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and /keys/query continues to return stale information, even after the remote user rejoins the room.

1. Have a room with one local user, @alice:local and one remote user, @bob:remote.
2. Have @alice:local do a /keys/query request to cache @bob:remote's device list.
3. Have @bob:remote leave the room.
4. @alice:local observes @bob:remote in the device_lists.left section of /sync.
5. (bad) If @bob:remote then updates their device list and @alice:local does a /keys/query request, a stale response is returned.
6. @bob:remote rejoins the room.
7. @alice:local observes @bob:remote in the device_lists.changed section of /sync.
8. (very bad) If @alice:local does a /keys/query request, a stale response is still returned.