Sorry for the delayed reply. Your question got lost in the cracks.
The [topic-url] tag will automatically be modified to force login and redirection if the forum is private via bbpress settings or via the Private Groups plugin.
Can you provide a sample of the topic URL you get in the notification as well as the support information from bbpnns Support tab? Feel free to mark the reply as private or to send it via email to vinny [at] usestrict [dot] net.