This happened in the last couple of weeks - level 1 and level 2 s2members can no longer see the original post in a bbPress forum topic, but they can see the replies. These level 1 and 2 members are Participants in the forums and can create their own topics, but now can't see their own posts.
Admins and level 3/4 members can see both the original topic and the replies.
The forum posts are restricted to level 1 or higher.
I found this (unresolved) thread on bbPress forums that describes exactly what's happening here:
I'm on bbPress 2.5.3 and s2Member Pro v140105.