Multifandom Doodle

It's unfortunately just the way AO3 works. If it's a newish fandom that hasn't had that place used in another work, and an exchange uses it unambiguated in an different slot (like character), if you want to use that tag in a free form slot, you'll now have to use it as Sesame Street - freeform instead of just Sesame Street. Forever, even in things totally unrelated to the exchange.