Even if using your fingerprint scanner stops working, you should still be able to use the touch screen to unlock your phone with your PIN or password. A broken fingerprint scanner will not stop you from logging in because PIN/password is always an option.
Streshi within grahpene there are some issues for this
What issues are you referring to? If there are any that are related to fingerprint scanning in GrapheneOS, I'm not aware of them, but maybe you know something I don't? This sounds like a hardware issue, not a software issue.