I have the same experience with a new fp3+: fingerprint sensor rejecting finger many times per second, until it locks me out. This is at genuine attempts to unlock, just by moving the fingertip in position.
It seems I can only successfully unlock by placing fingertip right in the middle of the sensor in the first attempt. Even then it tends to buzz a few times indicating several rejections
I’m also having the same problem with my FP3+ - it seems to have got worse since I recently updated the OS so now it is only exceptionally that I am able to use it to unlock the screen. Most times it locks out and I have to enter the pin code to unlock. Is it possible to reduce the speed or sensitivity of the sensor?