Does it prompt for fingerprint if you enter an empty password? If not, I have done a little looking around and found a source that states the following :

Add the following two lines to /etc/pam.d/kde :

auth sufficient pam_unix.so try_first_pass likeauth nullok
auth sufficient pam_fprintd.so

This configuration allows you to first press enter on the empty password field to be prompted for the fingerprint

@RegalPotoo
Maybe @kde knows about it?
@kde

So before: either enter password, or use fingerprint.

There was a bug where fingerprint always worked for me but password often didnt until I used it, quite bad.

Now this sounds as if you could enable "require password AND fingerprint" (both)