This has happened on every Android phone I've had including my current phone, a Pixel 7 Pro. My fingerprint unlock stops working after a few days and I have to reregister my fingerprints to get it to work again. Not specific to any one finger, happens on all fingers. It is not forgetting the fingerprints, they are still showing as saved. I think the subtle variations in dryness of my skin from week to week cause them to become invalid but I am not sure.
Note that this is not affected by my fingers being wet or dry. I have also tried registering my fingerprints when my skin is at different levels of dryness (e.g. summer and winter profiles), but that does not seem to solve the issue either.
What are the possible causes, and how to fix it?