To all who see this, I also had this same issue. And I fixed it in just less than an hour.
The fingerprint scanner on the Note9's and S9's do seem to not like moisture. I powered my Note9 off, and rested it on it's screen on a flat surface.
I then took 99% rubbing alcohol and a Q-Tip and poured drip-sized amounts of alchohol on the Fingerprint sensor. I wanted to make sure the fingerprint sensor was neatly dressed in the alchohol and allow the alchohol to dry there.
Waited about 15 to 20 minutes and then powered the phone on. Set the fingerprint scanner to unlock the phone, and tested this 3 days before posting here. I no longer get random reboots.
If you try this yourself, make sure you use 99% alchohol. 70% has water mixed in which the sensor may clearly not like if the issue is really moisture getting in the sensor.
I tried this after seeing another user in another forum claim that he used rice to fix his sensor rebooting issue. Rice works but it takes longer.
Do this at your own risk. Good luck.
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