DIAGNOSIS The memory backup circuit on the main control board has failed. This circuit uses a capacitor or small battery to retain clock and settings during power loss.
FIX The entire main control board must be replaced. The backup component is soldered onto the board and is not a user-serviceable part.
IF THAT DIDN'T WORK This symptom is definitive for this failure. There is no other user-accessible fix. A full control board replacement will resolve it.
HEADS UP While the clock reset is annoying, it does not affect the cooking functions. You can continue to use the microwave safely until you decide to replace the board.