On the reset password function, Username is case-insensitive.
On the login function, Username is case-sensitive.
So if you try to log in with the exact same username as you used to reset the password (i.e, with the “wrong” capitalisation”), and the brand new password you just created, it’s rejected as “Invalid username and password”.
Logon and password reset features are basic functionality for all websites. But still some people get it wrong…