The username 'at' cannot list on logon screen

[Kyle Cheng]

OS: Ubuntu 20.04.x LTS (Desktop)

Problem description:
When I try to use 'at' as my username, after rebooting, gnome-initial-setup
reappears and asks to create a new user.

Re-produce procedures (1):
1. Del all other users from terminal
2. Reboot to OS.
3. Add a user by gnome-initial-setup dialog.
4. Use 'at' as username and login name.
5. After setup finish, reboot.
6. When boot to OS, gnome-initial-setup will pop again for asking create a new user.

Re-produce procedures (2):
1. Create a user (e.g. username is 'user')
2. Check this user account can be showed on the logon screen and can login into OS.
3. Logout 'user' account and swith to terminal to modify the account with root permission.
4. Modify account 'user' to 'at'
5. After finish, reboot
6. When boot to OS, gnome-initial-setup will pop again for asking create a new user.
7. At this moment, switch to terminal with root permission
8. Modify account 'at' to 'am' and reboot
9. After boot to OS, gnome-initial-setup doesn't pop up and account 'am' is showed on logon screen.

Is this a bug or 'at' is a reserved word? I can't find any document relevant about this restriction.