At random times I awake in the morning to find my S8 had rebooted during the night and is waiting for my code to restart. This is very unfortunate if I have an alarm set, since is does not go off, and I cannot receive calls. I have attempted removing third party apps, safe starts and checkingn for any schedules that may have been setup, yet it randomly continues.
It seems you have completed a good portion of the troubleshooting already. I recommend having the phone evaluated by a certified technician. You can find an authorized service center using this link: https://www.samsung.com/us/support/service/