Please follow the confirmed thread on this issue.
Sendbird have changed their retention period policy and have stopped storing the messages that are older than 6 months. Luckily, the devs have already been working on an in-house solution for Chat and we have all of the older messages backed up in our databases and we are already recording all of the new chat messages as well. Once Chat 3.1 is rolled out, all of these older messages would become available again. We don't have any solution for this issue in the current Chat version because Sendbird doesn't show us any of the older messages anymore.