Bug with thread filter
If I recall correctly (which is fair to question, as it's been a long time since I looked at archiving threads), once a thread is archived, it is, by design, cleared of group limitations. That is, it may still have the Group flags set, but they are no longer active, with the intent being that a thread, once it stopped being relevant to the current events of the game, could be archived and those players who weren't in that thread could go back and read what had happened with the other players.
So, what you're seeing as a bug is actually an intended feature.