I’m experiencing the same or a very similar problem. After backing up, try disabling plugins. Did not help seem to help me but it’s a good place to start.
Second, run repair via wp > tools > forums > Repair Forums. First, remapped the users. I was seeing 0 Voices in the counts of larger reply threads. Then I ran all the Recalculate options plus repair buddypress group on accident, which seems to have changed the 0 voices bit to 1 voice on everything. Then run the Count options/repairs.
Seems to have help on my end… good luck.
