Login/Lock Screen showing memoji WITH USERNAMES and asking for "enter your password" instead of "enter your *username* and *password*"!

Go to Apple > System Settings > Users & Groups. 

Click Login Options. Change "Display login window as" to Name and Password. 

This should hide the list and require users to manually enter their username and password.

If you do not see that option in Users & Groups, have a look at > Hide a user account in macOS - Apple Support

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and toggle the Name and Password option on and off a couple of times. Reboot normally and test.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If you do not see that feature, have a look at > Hide a user account in macOS - Apple Support

Try Lock Screen settings, turn off "Show user name and photo" and change "Login window shows"