Ok, I won’t say I’ve fixed this bug, but I have managed a dodgy hack around it that fixes it sorta. In the immediate term, it has been fixed it for lemmy.world and lemmy.dbzer0.com users. I’ll look at other instances soon.

The answer is to re-pin the post, wait a while (10 minutes is enough, but I might try reducing it and experimenting) for that to propagate/federate across Lemmy, and then Unpin the post. Doing it this way unpins the post properly.

It appears the original unpinning for some dates didn’t get exported to other instances. I’m still not sure what was special about November and December 26, I got a bit distracted with the solution and not bothering much with the cause.