There's been an issue for a few weeks in the community forums where the last post as been wrong for some threads. For example in the attached screenshot:

Discussions are sorted by Last post, but it shows a thread with last post by me on 4 Mar 2025 ahead of one with last post two days later. The order is actually correct because "my" thread has a subsequent post by Tim Hunt on 6 Mar 2025, but obviously it's this latter post that should appear in the Last post column.