Having a bit of an issue with any limiting software when I set the master buss to Mono. 
 
It doesn't seem to matter if it's Boost11, Limiter6, or the limiters built in to the Sonitus FX plugins, they show they're working, but the signal seems to pass right through as if they're not.  I'll find myself +3 or 4 dbs.  When I switch the buss back to stereo, the problem goes away.

Sounds as if the Mono interleave is summing the two signals after they've been individually limited, which would give you a nominal 3dB boost. But I'm not able to replicate that. Multiple limiters I tried maintain output level regardless of interleave.

Only thing I can think is that maybe you have ProChannel active  on the Master and set to Post, and it's doing something after the limiter that is boosting a Mono signal more than a stereo one.

In any case it's likely a project-specific issue. I don't think it should matter, but what's your Pan Law?

   
   
  

That's normal operation when doing that. Mono is a single channel, so when you combine 2 channels into one channel, you will get a Db increase according to your pan laws. Nothing is wrong here.