Bad programming. People who have experience with embedded programming knows that reading out a button usually means denouncing. At the speed a microcontroller can read out a button it will change it's state multiple times per press because of contact bounce. Meaning when a user presses a button the program sees off, on, off, off, on, on, off, on, on, on, on, on, on, etc.

Now if you just naively read out the current state of the button and do something with it elsewhere in the program looping may be off or on randomly.

It is not hard to imagine if there is some other logic (or e.g. a rate limit) on the 30 seconds and on the beep that these would see different slices in time of the button. Congrats you built a button-debounce based RNG.

Physical buttons can be surprisingly complex if you don't rely on someone else's driver. The correct solution is to debounce the button, that can be done either in hardware (too expensive, so rarely done) or in softeare, by e.g. averaging the last 50 reads and wait till the majority is either off or on.

This should be common knowledge for embedded programmers, but every noe and then you will see someone who has never heard of it.