messages are flagged as non-received, when aborting a E2E session
Just close a E2E session. The messages you send are flagged as unreceived, until the correspondant answers (then, you sends yourself a urn:xmpp:receipts). Then, all the new messages are shown properly (however, the old ones are still flagged as unreceived).
The XML console shows that in the whole story, all the expected urn:xmpp:receipts are received correctly.