NianwenDan I am unable to replicate the issue.
I registered a new user named test_user on my forum without any issue. I logged out and was able to log back in by entering the username in the identification field of the login modal.
Do you see an error message? What's your Flarum version? Maybe it's an extension that adds validation rules to the login form?
I don't think Flarum performs any kind of validation on the credentials entered during login. The only test is an evaluation of PHP filter_var($identification, FILTER_VALIDATE_EMAIL) to choose if the value should be checked against the email or username column.
