Message encrypted with different session from what is sent to recipients

[uhoreg]

From a rageshake:

The recipient receives a Megolm session, say with session ID aaaaaa, and then receives room events encrypted with a different session ID, say bbbbbb. Room messages encrypted with bbbbbb only start appearing after aaaaaa is sent. Both aaaaaa and bbbbbb are for the same room ID. Room messages encrypted with aaaaaa are never encountered.

So it seems like room messages are being encrypted using a different session from what is sent out to the recipients.

Unsure what client the sender is using (could even be a non-Element client). Recipient in this case is using Element X iOS.