Maybe there’ s a speaker solo activated for the mono configuration…?
No, I thinks it’s the auto normalise? function being on by default on the Mono setup.
Just to add… The situation is now solved for me 
And just to add even more, you’ll only really notice this if you compare hardware mono summing against the control room mono summing!
So, where IS that configuration? I see where you can expose the presets…and already had that exposed…I don’t see where you edit their settings…
On the Downmix Preset Box (were you select Stereo or Mono) there is a little gray arrow just above the top right of the box!
Got it…thanks…and yes, that seems to have made them close enough.
Why on earth would that be the default?
Regardless…it’s also a little confusing how you change it…so, for anyone else reading, you can’t select the presets inside the MixConvert window…I mean you CAN…but, that’s not what you want to do UNLESS, you’ve selected the mono collapse in the control room FIRST…THEN you open the mix Convert window, make the changes you need.
Cool, it may be prudent to put solved in the header!
Done.
Just a little bump in case someone else has noticed this problem.