Device list not updated on sending device, leading to UTDs

[richvdh]

I now have seen twofour separate reports of UTDs where the logs show:

• Recipient logs in on a new device
• Sender does not perform a /keys/query request for the recipient
• Some time later, sender sends an encrypted message, but omits the new device
• Recipient (obviously) receives a UTD.

It is unclear if the cause is that Synapse does not tell the sender about the new device, or that the sender ignores the notification.

Something that is in common between the reports I have seen is that the sender and recipient are on different servers, so one potential cause is that the federated device-list-update-notification is getting lost. I've now seen a similar report between users on the same server.

The senders in the two cases are on different clients (EWR vs EX).