I have the same problem with my four year old GE, JNM7196SK1SS microwave. Couldn't find an answer online, so when I now microwave popcorn, I bypass the popcorn button simply set the cook time for four minutes.
Now.....you have to pay attention to this a bit. When you're into the 3rd minute of cooking, listen to the "pops". When you no longer hear popping for three seconds or so, it's time to open the door and cancel the remaining time, otherwise, you're likely to burn the popcorn.....
GreenBeretLTC
November 2020