I wrote answer in assumption that you create custom role with URE.
bbPress creates its roles on the fly, via code. So if you use some of bbPress filters to add your own role, it may be correct. If you use right filter, bbPress should show your role at its own roles drop-down lists. Try Settings->Forums->Roles to set bbPress your own default role for new registered users.
Or try to redefine bbPress default role using its ‘bbp_get_default_role’ filter:
bbpress/includes/core/options.php:535: return apply_filters( 'bbp_get_default_role', get_option( '_bbp_default_role', $default ) );