I actually got the following answer regarding my situation from the Xtnd.it team:
Hello Rick.
Unfortunately we can’t help you with the problems you describe, because there is no a WP phpBB Bridge bug. As you say the plugin working good. So what you have to do is to check the logged in users permissions somehow from within the WordPress. A plugin we use to check the permissions for our users is : User Role Editor (https://www.ads-software.com/extend/plugins/user-role-editor/ )
By creating user roles and assign to them the permissions you like you will be able to control you content visibility.
Kind regards
Xtnd.it Group
