This happened to me because I restored my boot disk from a full disk clone (thanks to a bad macOS update installation). I didn't want to restore to an unencrypted disk, so when I booted from my backup disk to do the restore, I used Disk Utility to encrypt my internal boot disk with a password.
After encrypting and then cloning the backup to my boot drive, I was greeted with a screen like this on my first boot from my boot drive:
Fine, no problem. I entered my disk password, and the normal login screen showed up. I entered my user password and everything was working...except I could not re-enable unlocking my disk with my user's password:
After searching around for how to fix failure of "Enable users" when "Some users are unable to unlock the disk", I discovered that there are a set of command line utilities for dealing with FileVault and I was able to fix the problem by decrypting the disk and then re-enabling FileVault.
Here's the command to decrypt: diskutil apfs decryptVolume /dev/disk1s1
To get "/dev/disk1s1", I simply looked at the info for my hard drive ("HD") in Disk Utility. (Normally this would be named "Macintosh HD", but I always rename mine.)
Then I simply re-enabled FileVault through System Preferences.
Warning: Messing around with disk encryption (or messing around with anything involving your boot disk) is risky. Always have a known good backup before you try anything like this, and proceed at your own risk – I am not responsible if you break your disk trying to follow these instructions.