If you visit the community on the original .world URL, there’s just today’s pinned thread as you’d expect. Viewing through lemm.ee (both web and Liftoff app) I see Friday’s thread still pinned too. And according to @thegoblin, their instance still sees Sunday’s thread pinned.
So I’m gonna say this is probably down to .world’s current overload and resulting struggle with federating everything properly. It’s a LOT better than it was before they upgraded, but clearly some things are still slipping through the cracks.