It was weird, I would unlock my phone with an alarm icon on the status bar and a black/blank screen. I noticed this about 30 min after it was supposed to go off.
Anything with an OS will have memory leaks, conflicts, ... etc over time. I prolly haven't rebooted my phone in weeks.
Not really mad, just found it curious. I remember all iPhones had a no-alarm issue years ago after going from Dec to Jan; it was in the news and a bunch of NBA players were interviewed about it.
It happens.