So I'm running KDE Neon, up to date, and when the screen locks I can't get the password prompt to show when I tap any key on the keyboard. When I tap the touchpad (e.g. mouse movement), it shows me the password prompt and I can input the password to unlock. This is after I login, and have been using it, and then the auto-lock kicks in at the time it should.

I'm using default theme, no manual change in any of the config files except through UI settings, etc.

It's a bug already reported here. You could go there and confirm it.

Reading through the bug, sounds very similar, except if I move the mouse I can unlock it. I'll chime in on that ticket, as it's likely related.