They were too cheap to put a thermal switch on the magnetron and/or its transformer. They made the fan loud, instead of having an intermittent buzzer, so you wouldn't forget the food was in there. Or some eco busybody established a rule somewhere in Jenn-Air's selling territory that mandated those features.

The Owner's Manual for Jenn-Air 27" AND 30" BUILT-IN CONVECTION MICROWAVE OVEN states, "NOTE: The microwave-cooling fan may continue to run for several minutes after the cooking cycle has been completed. The microwave cooking does not continue beyond the end of cooking cycle."

It appears that you have a different model and/or Jenn-Air decided that it was only necessary for the fan to run for awhile, not to also operate the turntable and oven light.

Update: The owners manual also states, "When a Microwave Timer is set, the WHEN DONE option will appear. This option allows you to select the microwave oven behavior at the end of the current cooking cycle. You can select between Keep Warm, Hold Temp, and MW Off. Keep Warm will run the microwave oven at 10% power for 60 minutes, Hold Temp maintains the selected cooking cycle after the timer expires, and MW Off deactivates the microwave cooking cycle when the timer expires.

When the stop time is reached, the oven will Hold Temp by default or behave as set in the When Done menu."

Are you selecting "MW Off"?

My unit doesn’t have that option, but i think MW off means Microwave off when cooking is complete, which is the default setting in my unit. when cooking is complete, the microwave cooking is done, but the cooling fan would continue to run till i open the door or i press ”Cancel”. the cooling fan is very loud.