Limiters failing in Mono mode. (Sonar X1 Studio)

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.