FWIW, I also measured the Atmega with the front panel's polarity switch in the other position and the readings are 2.6 V and 3.8 V.
So the higher of the voltage readings dropped a bit when the polarity was switched but even then the Atmega is outputting 1.6 V more than when the exact same cable and sensor are attached to the ARM.
I wish I could make some sense of these numbers. I can only surmise the low voltage is causing interference/instability on the ARM. I suppose it can't be a firmware issue of CC reduction handling on the ARM because others are evidently not experiencing this. So something must be wrong within the ARM's hardware. All I can tell is nothing is rattling or doesn't look loose inside but that's the limit of my "expertise".