The issue WEB-2702 has been around for 3+ years now
i know this is a duplicate but i feel like the web team just forgot about it and the issue WEB-2702 says
"Resolution: Fixed" 
which clearly isn't fixed yet
at least you guys can make a exception list we can request to add username or some better idea by The team, Like i said i feel like the team just forgot about it, So at least report the team about it.

i brought the game yesterday and not being able to use my username frustrated me
My username: TamimZaman99 is also being affected, which I'm using in bedrock edition but unable to for java profile