After the matrix.org breach, there were a lot of UISIs for matrix.org<->elsewhere E2E rooms. Circumstantially we're assuming that this servers ended up with an inconsistent view of the devices present in a room.

Possible causes are:

• If an m.device_list_update EDU gets lost, servers will be stuck with the stale list until the next m.device_list_update from that server, which happens infrequently.
• However, EDUs should not get lost; we should retry m.device_list_updates until we get a 200, so it seems there is a bug here.
• If we see an unrecognised device from a server, should we not re-sync our view of that server's device list anyway?
• In theory we re-sync our view of the device list when we see an m.device_list_update which refers to an unknown previous m.device_list_update. But this might not be working?

(See element-hq/element-web#2996 for the main UISI bug)