@ti30xa
Can't find a wiring diagram for your model but with a lot of microwaves there's a power relay on the control board that is operated to supply power to the high voltage circuit i.e. magnetron, HV capacitor, HV transformer & HV diode and it is released at the end of the cooking cycle, so it could be sticking (stuck?) relay contacts.
This is given that as you say the magnetron is turned off when the door is opened. This is because the primary and monitor door interlock switches have released and operated (respectively) to disconnect the power to the HV circuit and also short circuited the circuit so that it can't operate.
The control board part # is WB56X21278 but it ain't cheap.
You could try locating the relay and just replace it, if you prove that its contacts are permanently operated that is. The relay number should be printed on its case.