These numbers are very confusing. GRBL-M3 and GRBL should give you the exact same voltage at peak. Not clear to me why you’d be getting different values here.
It’s interesting that the voltages for M3 do seem to scale based on power level although at extremely low values.
Can you confirm that your meter is known working properly? If you check the 12V pin does the meter in fact measure 12V?
Also, try this, test voltage at pin D11 and Gnd of Arduino board at varying power % levels in LightBurn.