The majority of those microwaves pulse the power on and off at intervals, sometimes very long intervals (measured in seconds!).
Good microwaves, with an inverter (see the thread above!) actually adjust the power being output. You can evenly heat a cup of liquid to any desired temp using these microwave ovens. They are well worth the few extra $s (although mine actually cost less than a fancy smart microwave!)
The power setting just pauses the power for % time like your doing manually. I do 70% for my oats in a big bowl so the bubbles die down before the power repeats.
I would have to try that but since I never measured exact amounts, I preferred to pause upon visual inspection, as in: as soon as the milk started bubbling, I stopped and stirred.
The typical workflow is: power button > number button 1-9
My daily driver is a bit more fancy, and has a single button to cycle through the available power settings.
For your milk oats I’d suggest a power setting around 30 to 40% and increasing the cooking time by a factor of 2.