I guarantee you this problem can be reproduced just by clicking the Login button and entering your e-mail in the "Forgot Username" field. Check your e-mail and it will return your Summoner Name instead of your Username.
EDIT: Also, the reason exact reproduction steps are not required is because the query for your username is server-side and thus has nothing to do with what I'm running.
[Website] "Forgot Username" returns Summoner Name
EDIT: Also, the reason exact reproduction steps are not required is because the query for your username is server-side and thus has nothing to do with what I'm running.