It seems that adding a new user through "System Settings" -> "Account Details" -> "New User" doesn't work: I fill in the new user's details and when I click "Apply" it turns back to grey and nothing happens. 

Running system settings from a console, it gives the following output

> systemsettings5 
log_user_manager: 1000
log_user_manager: Checking password
log_user_manager: Verify password is empty
log_user_manager: Checking password
log_user_manager: Verify password is empty
log_user_manager: Checking password
log_user_manager: Quality:  31
log_user_manager: Saving on Index:  1 
> 


No sure when it first appeared. I just noticed it in KDE Neon user edition having latest packages installed (as of December 18th).