No hot elements in the microwave unit, and it has electronics to provide variable energy adjustment; so no on/off action. Just like your microwave oven has a power level setting. It measures the water temperature and adjusts the magnetron output to maintain the exact temperature commanded.
Thermal overload is the water reaching high temps in a short time almost causing it to boil. It's not caused by the heating elements but by the heater itself applying more energy than there is water. It shuts briefly to protect you from scalding and the heater from blowing up. The microwave will not use heating elements but I don't doubt that will suffer from thermal shock, it all depends on the controller being used.
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