Has anyone had one of the metal buttons fall out of their PID on the M58? Is it safe to glue back in?

No, i would not glued it, because if some glue goes in the microswitch it will clog it, and then you have to buy a new PID board.
Just try to fit it again, pushing not too hard, if not remain in position just put a thin foil of something ( preferably non conductive) inside the button hole, then try again.

yea that's my worry as well. It sits in there most of the time. Thanks for the foil idea! I'll try something like that.

Mine falls out occasionally. It's rare enough that I just put it back instead of thinking about a real fix. Maybe a little wedge of two-sided tape?