Nonexistent account on lock screen

When you create an MSAccount-linked user account, the actual username used by Windows is a shortened form of the 'Full name' of that MSAccount, usually its first five characters.

So it sounds as though the additional entry is not a phantom but an illogical repetition of your intended account.

Please do this to guide future responses:

1 Right-click on the Start menu,
Select Windows PowerShell,
Enter this command net user

2 I expect that the results will show the 5Letter user [but not the full name that you expected to see] so enter
net user <5LetterUsername>
i.e. that username returned by the earlier net user command.

3 In the results, look down the list to find its FullName entry. I expect this to be the FullName you used when creating the MSAccount. Please state whether or not this is the case.


Denis

It is always worth having a spare, password-protected, local, Admin account in case your day-to-day one gets corrupted. You can then use it to get back into the computer whatever has happened to the day-to-day one. And write its password down somewhere secure yet accessible - further notes in my ditty - Create two spare local, password-protected Admin accounts [post #25] - TenForums.

I suggest you create a spare, password-protected, local, Admin account before you convert this "have switched to a local account with no password" one to an MSAccount. I don't suppose anything will go wrong during the conversion but having the spare one set up beforehand will ensure you can recover from any problems.
Switch to Microsoft Account - TenForumsTutorials

It was not a phantom account so the procedure you found online was not applicable. When you next need to search online, try these first
Tutorial index - TenForumsTutorials
Advanced Search - TenForums

All the best,
Denis

Everything completed and back to Microsoft login, but I have the same problem as before. 2 accounts are showing on the lock screen again, one correct and the other showing "snick." I ran "net user," but the snick account didn't show up. However, there is a default that's probably it. This is what I got:



That's apparently Windows Defender Application Guard, but it's not checked in Programs & Features > Turn Windows features on or off.

EDIT: All seems to be in order now. Thanks for your help, Denis. The problem seems to have been with User Accounts (settings by running netplwiz or control userpasswords2). Once I got that out of the picture, the extra account went away. When I started to reset for passwordless logins, the username popped up as "snick." I changed it to the correct one (email address) and everything fell into place.