User not visible on Users & Groups

This was a side effect of one of the bad security patches for Mohave. If you update to the latest security and supplemental updates it will fix the problem. However, any/all accounts that were created prior to the update will be inaccessible, and invisible going forward. I had to kill the 'bad' accounts in terminal after I ran all of the updates.

I cannot find the Apple article which mentions moving the user account to another location (I recall seeing a reason why the account would be at a different and perhaps hidden location, but I don't recall the reason for it --- this is different than simply changing the home folder location in the "Advanced Options" of Users & Groups).

You can see if the "hidden" user has the "hidden" attribute set for the account by using this command (modify it for the exact name of your user including the path the to that user's home folder):

sudo  dscl  .  read  "/Local/Username"

This will show you the options/attributes which are currently set for this account called "Username". The following command will only return whether the "IsHidden" attribute is set:

sudo  dscl  .  read  "/Local/Username"  IsHidden

If the "IsHidden" option was never set this may return an error or the "IsHidden" option won't be shown in the output of the first command I listed. The macOS default does not have "IsHidden" in the output.

Here is an Apple article which shows how to hide the account and how to make it visible again:

https://support.apple.com/en-us/HT203998

Here is another Apple article on how to modify the user name and location of the home folder, but I don't know if I would do this since there may be something else at work here which moved the home folder originally (I really wish I could remember what triggers this behavior):

https://support.apple.com/en-us/HT201548

Edit: Make sure to have a good backup before attempting any changes.

I've only used these commands to unhide a hidden user account. I really don't know how macOS handles these things or what other things may be involved since it is so hard to find general information on this topic. I do know that if a user is associated with a network account (aka Active Directory login) or perhaps some macOS privacy feature that the physical location of the user's home folder may be relocated as part of the process. Perhaps there is another value similar to "IsHidden" that controls whether the user is displayed within the User & Groups System Preferences.

Is this Mac being managed by a school or business?