While the screen is lock and then you press the fingerprint button before the animation starts the screen flash., this happens when the screen is black, this does not occur in lockscreen mode.
I hope you can fix this.
App is not compatible with the device?
I found a way to fix it..
Just follow the steps from this site.
then, wait for it. until Play Protect Certification says "Device is certified"
I think you need to wait at least 30 min...