Update: This only pertains to closed topics, regular ones still display in the freshness column.
I’m having a similar problem, none of my topics seems to show up under freshness in the forums root. It just says “no topics”, the other counters for topics and posts seem to be working fine though..
Anyone got a solution for this? I’ve used bbpress for some two weeks now and it’s been this way from the start, I’m using the newest versions of both WP and bbpress.