Why does macOS 10.13 suddenly asks for a PIN instead of password for login?

You have your Yubi key in the USB port at some point, put the Yubi key in and enter your Yubi Pin this will unlock your screen

I run into this, too, with Big Sur (and earlier versions of MacOS). I've found that I can work around it by

• clicking the "Switch User" thing at the bottom of the screen
• clicking my face when it appears At this point I'm presented with a proper "Password" entry, that works fine.

However:

Some other prompts don't let me work around it that way:

• Prompts from clicking the lock in System Preferences) let me use Touch ID, which works fine; but
• Prompts which pop up when I try to install software that needs Admin access have no such workaround. I assume that, to install that sort of software, I'll need to fully log out and log back in (I haven't yet tried that).