Thanks @Jim M for your support. I just tested this by removing the potential conflicting category. The filters still do not show. I also tested in on another section where I have just two simple subforums (BMW and Mini). But the filters do not show.
Interestingly, if I use fluid view on a global scale, the filters do show, as designed.
If actived per forum, it merges the topics, but now filter is displayed.
Edit: Just tested it by disabling third party apps. And it worked! So I will check which one it is...and found it: The app "Different sidebars for each forum 1.1.0" is causing the conflict.