Bus level problems in mono... help - bug?

Hi all,

I have come across a somewhat strange behaviour in Sonar 5 PE regarding the levels on the busses...
I'm running BFD as tracks and these output to a "Drums master" stereo bus with a Sonitus Multiband Compressor acting as nothing but a limiter.
The "Drums master" bus outputs to a "Master" stereo bus...
Now if I want to set the "Master" bus output to mono so I can listen for phase problems the "Drums master" bus suddenly overloads it's levels (goes into the RED area of the meters and goes beyond 0dB...

It's like the "Drums master" also turns into mono but AFTER the effects bin, thus increasing the overall level... - if I turn the "Drums master" bus into a mono output all is well - this is why I presume that the Multiband Compressor now sets the correct output level because it "knows" that output should be mono...

So HOW can I reliably trust this setup when distortion might be introduced from this behaviour?

Thanks for you time!
Dennis P