I encountered this problem too and it seems that when I add a topic in a forum under my test group, it works. But if I add a topic through the top-level Forum tab > New Topic, it doesn’t work.
Maybe that New Topic button should not exist on that page? Does that make sense?
Is that Forums tab supposed to show only public forums that are not associated with a group? Because there are forums that fall under groups too. It’s kind of confusing…