Message unpinning itself?

I've been running Discord servers for a little over a year and a half now (I think) and I've never encountered anything quite like this. I pinned a message in an Announcements channel way back, and at a random point I found it wasn't pinned anymore. So I re-pinned it and it stayed pinned for a few months, but a while later I checked and it had unpinned itself again. I've already re-pinned it again and have gone through my server's Staff team to make sure it wasn't one of them pulling a prank, but it looks like this may be a bug. Anyone seen this before or know how to keep it from happening? Thanks!