I have pre-ordered SGS9+ and FPS is faulty since day one. I have read Android forums and tried all solutions along with disabling gestures.
Sadly it still behaves like pice of sh!it.
Cheapest Xiaomi that lies on my shelf has blazing fast sensor that never failed. Samsung one rarely works, finger recognition is below any criticism.
And yesterday's update haven't fix anything related to sensor only the face and iris. Seriously I'm waiting one more week and I'm returning it.
Check your setting of screen lock delay in Secure Lock Settings. There is an adjustable time delay before the phone locks after screen off. Mine is 5 seconds which can be adjusted up to 30 minutes. You can set it to "immediately" to set the phone to lock immediately when the screen turns off.I've had my S9 since Saturday, had the Note 8 before. It was a great phone and FPS always worked. Now this S9 sensor doesn't always work to unlock the phone which is disappointing. It seems to me that the phone has to be locked for a certain amount of time and then it will unlock.