Olde time microwaves had a moisture sensor inside which allowed them to sense when popcorn was done popping, automatically. Really fancy ones have a microphone, and will listen for when the popping is done.

But lots of microwaves literally just throw on a popcorn button that’s just some arbitrary preset time duration. These do not get consistent result, and as such, popcorn makers just tell people to not use the feature at all as they can’t guarantee results.