I tried to help out with a suggestion. If you think you have configured your forum as you like it then let it be. The forum online box is only showing who is browsing adventures log. That's it and that's never gonna change in woltlab software
It is logic to show only the users browsing per forum. If you are in the overall forum landing page it shows everyone who is online that's the difference.
You could remove adventure log category and have the invisible forum still showing.