I just got off the phone with Apple and they suggested running this command in terminal: sudo fdesetup remove -user Guest. It will then ask for the administrator password. Afterward, reboot your machine and the account should be gone. It was gone from my 2017 MacBook Pro and another 2016 MacBook Pro.
Edit: They are still looking into this issue and will provide an update to remedy, but for now they suggested users should just run that command.
Thanks! This worked for me.
It was disconcerting having an account with a seemingly unknown password on my system. Kind of glad it's just a macOS glitch.