I know it sounds bizarre. The fingerprint login success rate started decreasing over the course of 2 days until it wouldn't work at all. Happened even after re-adding the fingerprint multiple times, restarting, waiting it out for 1 full day until it became impossible to fingerprint in at all with the right thumb.
I have a protective screen but it has never given me problems.
Solution: added 2 right thumb finger prints. Works perfect now.
I wasn't deliberate in trying to isolate the issue. Just wanted to get it to work for now. Let this be a data point. If anything, adding two fingerprints for your main login finger MIGHT increase the login success rate if it's failing too much.