With bbPress the only way to do this is with “private” or “hidden” forums
• Public – Anyone can see these forums
• Private – Only logged in registered users can see these forums
• Hidden – Only Moderators/Admins can see these forums
Are you using private forums?
If not maybe your using a membership type plugin to prevent users who are not logged in from seeing the forums?
Follow This link. Just a simple snippet.
https://vvcares.com/w/wordpress-hide-pages-for-non-logged-in-users/
The snippet will do:
IF the user is not logged-in..
AND the page name is ‘members’ OR ‘activity’ OR bp-profile-page..
THEN it will redirect to REGISTER page.