i have not used it since park hopping coming into effect- not sure anyone has- not sure it's there YET
there were some FP+ we didn't use when we were there, it just sort of ends up in cyberspace. no harm, no foul. next day just use your FP for that day. (although i will admit, that some rides we went to use our FP+ and line was short so we just got on and didn't use it. we were sure then to trade it for any FP for later in the day rather than just lose it, in case we wanted something later, we still had one available to try and get something with it)
with park hopping you supposedly will get a whole new set (or just one at a time- not sure) when you arrive at the new park, and you get them from a kiosk, not MDE app, so it doesn't matter if you used all of them at the first park
but there is so much unknown at this point about 4th rolling FP and park hopping FP, but that is how i understand it so far