This happens sporadically but I've noticed that every once in a while (it just happened to me and it hasn't happened for the longest time) the alarm on my iPhone will go off internally but it will not make a sound or vibrate. What I mean by this is that if I press the home button (after waking up late because it didn't ring) I would see the alarm screen as if the alarm is playing but there would be no sound or vibration. I've noticed that this only happens to alarms that I set manually on the spot. It does not happen to alarms that I have set to repeat on certain days like every Saturday, Sunday, etc.
I would see the screen in the pic below when I press the home button but there would be no sound or vibration
I would see the screen in the pic below when I press the home button but there would be no sound or vibration