I hit the same problem immediately. Found this post trying to find a solution to the issue. Posting here to commiserate with you and should anyone else post an answer.
FWIW - I wanted to sub to the toplevel category anyway which also doesn’t work. My setup was simple:
Category: Help
Sub-Category: General Questions
I expected if I sub’ed to “Help” and then made a topic labelled with “General Questions” it would work fine since that topic also gets the “Help” category added but that didn’t work.