This is a new behavior that I just noticed. Might be something that the mods want to take a look at. Certainly useful for some pins, like the review the User CP one, but might be less desirable for things like the Forum Rules. Users can override this and manually re-pin it if needed. Unfortunately it doesn’t appear that users can pin any thread they want though, that would have been nice.
Go to Preferences >> Interface and see if Automatically unpin topics when I reach the bottom is checked.
I can check that, but that’s not really my point. I suspect a lot of users will want pins to disappear. I wonder if the Mods however don’t. Not sure if there’s a perma-pin post type they can use to make sure that highly important threads can’t be unpinned or otherwise hidden.
There’s Pinned and Pinned Globally. I think that both can be overridden by the user. Whenever I need to find one, I go to the forum and use the search function (going to the forum first lets you search just that forum.)
ETA: Oh, and Atwood suggests opening an incognito mode in your browser to view the forum, since that takes away the user’s settings.