Pinned Posts not staying pinned?

There are a few posts that are marked as “pinned” such as List Status for 2021 - Updates

If I click on the pin to make it pinned, when I click on the subforum, it is no longer pinned. Is there a way of making the pins stick?

Think I might have an idea why this is happening. If I change the pin at the bottom of the thread, it seems to hold. But when I scrolled up to the first post, the pin at the top was showing as unpinned, and sure enough, the thread was unpinned again.

If I changed the pin at both the top and bottom of the thread, then it seems to hold. Weird!