DIAGNOSIS The under-cabinet LED light strip is failing. On this model, it's an integrated strip, not a standard screw-in bulb. Individual LEDs can dim or die over time.
FIX Yes, you can replace it, but you need the exact part. The component is officially called the "LED Lamp Assembly." You'll have to order the specific replacement part for the ME21M706BAS. To install: Unplug the microwave. Remove the two screws on the vent grille (underneath, front edge) and lower it carefully. You'll see the light strip mounted to the grille assembly. Disconnect the wiring harness plug, remove the old strip, connect the new one, and reverse the steps to reassemble.
IF THAT DIDN'T WORK If a new LED strip doesn't fix it, the issue could be with the power supply to the lights on the main control board, which is a more complex repair.
HEADS UP When shopping for the replacement part, the model number is critical. Generic "microwave LED strips" from hardware stores will not fit or connect to the proprietary wiring harness in this unit.