Apparently, LL seems to know it's a technical issue and wants to investigate / is investing it, but wants residents to file support tickets on it. This came on SL Grid Status
Old IMs reappearing in chat
We have had reports of old IMs, some as old as 2 months, reappearing in chat as if they were just resent. We want to assure residents we believe this is a technical problem and not associated with anything intentional or malicious. We are investigating the cause and if you wish to send in a support ticket with your experience we will pass that information on to the investigative team.