I don't know whether to call it a bug or something, but I wanted devs to know about that.

I couldn't make my fingerprint reader unlock anything on KDE Neon, but I could use it and verify fingerprints with fprintd. I tried LightDM because sddm is supposedly broken, but still no change. Then I tried gdm out of curiosity and apparently it makes everything work, including sddm(!). I don't have to even use it, but I need to have it installed for sddm and everything else to unlock with fingerprint, like for example pkexec, sudo, or Plasma login screen.

And what's even funnier, settings -> user still displays that "Logging into the system with your fingerprint is not yet supported."