The newly created user is not created on /etc/passwd nor is displayed on User Manager after finishing creation. In previous versions, the new users weres created. The only issue was that the user with uid 1001 never appeared on User Manager but all users were created on /etc/passwd. Same issue on a fully updated Arch box.
Manually creating 4 users with command: sudo useradd -m -G wheel,storage,power -s /bin/bash mytestuser - create 2nd user. - is shown on User Manager (UM) right after creation but on UM close and reopen, the user is no longer visible. It is present on /etc/passwd with uid 1001. - create 3rd user - is shown on UM and is present on /etc/passwd - create 4th user - is shown on UM and is present on /etc/passwd Adding for example, the user Real Name using UM, triggers the dialog for sudo/root password and the change is seen on /etc/passwd Removing user with UM works as expected(removing user from UM list and passwd file).
Command mentioned on the previous comment was from my Arch box. Using adduser on Neon will provide the same output. uid 1001 user is available on sddm for login, on both Arch and Neon.
The only way to get a new user using user-manager is to check the box admin for this user. In that case polkit display the window asking for your password and the new user is created with the expected rights. Using Maui with Plasma 5.8.3
*** This bug has been marked as a duplicate of bug 373276 ***