Unfortunately, Meridian support was unable to resolve this. They've given up.

All evidence I've found points to some sort of problem with the .net framework tool they've based Control:Mac on (MonoTouch), but without Meridian's help in pursuing this, I'm dead in the water.

Sorry I couldn't contribute something more productive to this issue.

Living room: 818v3, Parasound A23+, Maggie 1.6
A/V room: 861v8, DSP7200SE L/R/C, 4x DSP320 surrounds, ID41 + Roon, UHD722, Prime+PS