I’ve run into several microwave controllers with a similar misfeature. All used the same controller board despite being different makes and models. The part that controlled the light was a plug-in board on the controller and a short in the light socket could blow a diode on it. Bad design since burnt out incandescent bulbs often fail with a short. The plug-in replacement cost ~$8, but nobody seems to stock it, only The whole controller board at ~$175.
I took out the power controller board, unsoldered the two resistors that burned and put on higher wattage rated resistors. The light sort-of works now: it makes several seconds to turn on, dunno why.
Probably the right fix would be to replace the board with a custom design with just a little transformer and a diode.
About the only thing it's useful for is to remind me when its preheated.