My S9 was shutting off, at first what I thought was randomly. The cause was a combination of the auto-restart setting mentioned in this thread, (which was turned on and set for Fridays at 3 AM, but not by me) , and the using the fingerprint scanner for lock screen. Since I need the fingerprint scanner, I turned off the auto-restart. My wife and I both have S9's. She doesn't use the FP scanner, but uses a PIN for lock screen security and the auto-restart works properly.
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