Oops, I forgot to mention some details. After restarting, I'm always asked to enter the SIM PIN, and the phone's PIN. Afterwards, I can unlock the phone with my fingerprint.
This time, however, after restarting I've entered SIM PIN, and instead of phone's PIN, I had to use my fingerprint, which wasn't supposed to happen.
I'm sure of this and am not misremembering.