The summary in this bug is something I have observed, too. In Plasma 6.3.x, after locking the screen, the password prompt is displayed immediately and the power LED next to my fingerprint scanner turns from white to green to indicate that it is ready and waiting for me to scan my finger. In Plasma 6.2.x, it would show the lockscreen wallpaper with the current time and only after I hit a key or clicked somewhere would it show my user picture, a password entry field, and fire up the fingerprint scanner.
This might actually be the root cause and the fingerprint scanner not working may well be just a side effect. IIRC, when libfprint is waiting for authentication, there is a timeout, maybe 20 or 30s. After that timeout, it turns the fingerprint reader off and requires a password. Maybe this is what happens here: I lock the system, it waits for authentication right away, and I can unlock with my fingerprint again. However, if I step away for a little longer or it locks automatically after some time, when I return the fingerprint scanner is not working. It is just a theory, but maybe this is because the scanner activates automatically when locking, reaches the timeout, and then is turned off and the system requires a password to unlock.