On Meta I get the same behaviour if I try to sign up with a taken email, though it’s a little janky. It shows this at first:
But if I refocus the input then tab away form it, I get the expected:
Here is the signup email checker on the input:
However we don’t seem to check it on the invitation page, it only checks the username via the UsernameValidation mixing:
So I think we need to add the check on the invite page, however we need to be careful because SSO can be used there as well and that has its own external email validation logic.