This is actually due to fingerprint reader being dirty. You either touch your phone with wet hands or use alot of sanitizer most likely. Use a wet qtip and lightly wipe the finger sensor. Use the dry end of a qtip and wipe to dry the sensor. Repeat until all residue is cleaned off.
The reason this is happening is because residue build up mimics a constant touch of your finger.
For some reason note 9 turns off when you put your finger to the sensor while the screen is on. Hold your finger on it while turning off the screen and keep holding finger for 20 to 30 seconds longer. Maybe an unintended bug?