I have no idea why your username doesn't work.
Please check you are able to get the expected results by going on the e621 website and typing "favoritedby:yourusername".
The other usernames test show that the feature seems to work for other users, and it is weird that it wouldn't work with yours.
Just a guess, but if you changed your username, maybe you need to enter the username you first used when signing up. It is up to their implementation at this point.