Same issue. Can shut down two or three times a day. Often within a minute or two of closing and putting in my pocket. I only discover it this quickly because my galaxy watch will report a disconnection from the phone.
I have tried the reset, which only causes the pain of having to redo all the hard work of setting up a phone all over again. Behavior was identical. As for latest software update, it's been two months and counting since the last. Weren't we promised monthly updates?
Also, my notification bar is non-existent 80% of the time, tapping icons and close widgets in the top corners of the phone are an exercise in frustration, handling receiving phone calls is sometimes a battle with the phone to let me answer as the screen changes 2-3 times before I can get to the answer button, and my charging port finally developed the dreaded hairline crack, despite extreme care taken. Perhaps the rotation sensor just isn't that well calibrated, causing most of this behavior?