I don’t have this checked, but Discourse is still autofilling the username fields with the first part of email addresses when people follow an invite link.
I tried unchecking the ‘full name’ autofill option too, but that doesn’t help.
Having looked into this more: I think this only applies to people signing up after receiving an invitation email, so that might be where the confusion lies. As in, I think the setting was intended only for public registrations, but people like me assume it applies to all registrations.
It would be great if we could stop this happening with people who get invited via email too, but if not, I guess a workaround is just to remove the ability for anyone to be invited via email.
We’ve made an update to how usernames are automatically suggested, and I believe we’ve resolved the bug you reported. When you invite new users, the suggested username will not be based on the email address.