I have the same issue. I've had this phone for less than 3 months. It will be sitting idle on a table not being used then suddenly I will see the white and red Hello screen and the phone light up out of the corner of my eye.
The 1st time it happened (less than 2 was after purchase) I didn't think anything of it, but since then its restarted every 3-4 days at most. Different battery levels, all while being idle. I did the troubleshooting and resets and Verizon had me send it to a local Samsung authorized repair center. They said all the diagnostics were fine, but they would replace the motherboard and the battery. They did that and same thing, 3-4 days later it happened again. I called Verizon back for the 4th time and they finally sent a replacement. Now I've had this replacement for the 4th day, and it just happened to me with this new one a couple hours ago. I'm so frustrated! (I've owned 5 different Samsung phones myself, not including my children's phones over the years so I'm very familiar with them). I decided to upgrade to this one from a Note 8, which I've never had a problem with. I don't know what else to do because I'm stuck with the phone, and I actually love it aside from the random restarts all the time.