Log In Information Glitch

When I was logging in, it asks me for a username. I already created a username a long time ago and I used the information that I usually use to log in ( email and password ). Then I logged off and tried the second time. It happened again, it asks me for a username like before. This is my first time experiencing this problem. Though I'm not sure if anyone else experiencing a similiar problem to this. I dont want to change my awesome username in case you were wondering " why not change it ?" Please respond to me as soon as possible if you find out the case

Put a step by step guide on how to trigger the bug here
1. Log in with the same information that you are currently using ( that already created a username before)
2. Asks for a username