hmmm..
ok, let me try and unpick this to help you.
buddyboss is a private development that includes bbpress and buddypress in it, buddyboss has its own theme, and is designed to work with that. it is really designed as a paid solution with its own theme. https://www.buddyboss.com/
Buddyboss is not a replacement for buddypress and/or bbpress – these free plugins are just used by it.
As the twenty twenty support says in their first post ‘I used the steps above to try and reproduce the issue you described, but I was unable to. When testing, I see the list of forums available as expected.’
bbpress works fine with twentytwenty, as indeed does bbpress and buddypress work with twentytwenty.
I suspect this issue is with using Buddyboss with the twentytwenty theme, and buddyboss may well consider that to be an issue with twentytwenty and not their problem, just as Ford might consider you using a Chevolet gearbox on a Ford and not working to be a Chevrolet issue. Since buddyboss is a paid solution, I cannot test to find out what is wrong.
I suspect the issue is with the default template that something is trying to use, and might be quite simple to fix, but as I don’t have access to Buddyboss without buying it, I can’t say.