On Ubuntu MATE 23.10, when trying to add a user through Mate User Manager, uppercase characters are accepted in the "User Name" field, but the validity check then rejects the user name based on its own regex and throws an error message: "running '/usr/sbin/adduser' failed: Child process exited with code 31".
This happens because Mate User Manager expects accountsservice to call useradd, but the patched deb version calls adduser instead. Some help may be needed upstream to patch the mate-user-admin deb package and make it use the adduser NAME_REGEX :