i have tried on other default wordpress theme, the mentioned users notification does not work on any of them.
i actually checked the notifications page, there has no notification for the mentioned action. so i dont think it might be a theme related issue. it seems like the notification event is not recorded in DB.
so to recap of what works and what does work:
the notification does not work if I mention someone in forum reply or in a forum topic.
Notification works for when i mention someone in buddypress activity streams.
thanks for you reply.
i thought that is buddypress feature so i asked here.
i have also posted the same thing in bbpress now. hopefull will get an response.