To answer your question, yes I did have the always on feature on.......
Full story:
So yesterday, this happened to me. The phone spontaneously went black. The battery was at about 48% when it died. I was having absolutely no luck getting any responsiveness out of my phone at all. However, my family has this app called "Life 360". On that application they could see that my phone was active and charging. But if you called it, it went straight to voicemail.
My phone was plugged in the entire time. I followed the "volume up, volume down, power" routine a few times with no luck. Then, I did the apple support chat. About 20 minutes into the chat, my phone spontaneously turned back on. It was then at 55% battery. I haven't had an issue since.
I was using the "always on" feature that is the default setting. I did do the software update as soon as my I got my phone. I have absolutely no idea what happened and neither did the tech on apple support.