Same issue here. I believe I found the issue. When the alarm goes off and I turn my face in the direction of my phone, it automatically goes into that silent mode and stays there. The screen still shows the alarm is going off but no sound. My theory is when I roll over in bed, not even close to being awake, the phone recognizes my face and puts it in this mode, which obviously causes significant issues bc I’m not awake enough to even register that the alarm is going off.
Last night I put the phone on the charger on my night stand and then turned the phone away from my direction so it couldn’t pick up my face - and that worked. The alarm went off normal.
Just playing with the phone and the alarm, if I set an alarm and it goes off, it continues to go off until I hit the button. However, when the alarm starts going off and I put my face in view of the front of the phone, the alarm goes into that silent mode without me touching anything.
Try turning your phone away from you at night so it can’t perform the face recognition or turn that feature off.