What are the possible causes of "That sign in name is not available"?

I know the obvious cause of this is a user trying to duplicate a name already in the system. I suspect that this message also is caused by username filtering, but I've got a strange one on my hands and I'm looking for other possible causes...

Issue: User with unique name and email address is trying to create an account via SSO. Other users are able to create accounts so I know it's not specifically SSO related. I, as site admin, go to Add a Member to create the user in the system manually (to link with SSO) but also get a toast notification "That sign in name is not available". Checked the database directly for any sign of that user's username... none. And of course, the GUI search tools show no sign of this user either.

I can't see that this user's preferred username is tripping any of the filters, but I would have thought as an admin creating an account, it would have overridden those controls if it were.