Usually when someone leaves the convo it can be gone.. but not usually both ends I've had convos go but they were from people who are now banned
Could be a bug, I've had a few people ask why I'm not following them anymore when I didn't unfollow them in the first place. If the admins run diagnostics/bugs check, they might come across any bugs/malware etc.
I didn't do anything. Someone may have been working on the database and some data became available again, maybe the server storage was being defragmented and some data wasn't available for the duration, maybe the database was repaired/rebuilt by the software when it detected errors, as you were clicking on conversation links which were broken. Who knows. I think it was a software glitch.