Funny. I was about to post the same thing this morning. Sometimes it goes off once, maybe twice. Sometimes, not at all. Very frustrating as I've been using my phone as my alarm clock for a while as well.
All of my settings were imported from my S7E. I wonder if that has anything to do with it.
I double checked all of my settings and even tweaked a few in hopes that would work. Nope.
This morning, I don't recall it going off at all. Even when I used to sleep through it, there would be a notification where it says snoozed where I could dismiss it.
I am going to blow out everything and start over and see what happens.