If client has Lastpass running on a browser, there's a potentiality that it fills in a password form behind-the-scenes when creating a new user, even if the checkbox is selected to email the user a link to create their own password.

If the client uses a browser without Lastpass, they have no issues creating the user.

Unchecking the box fills in a password automatically, so this shows that something is being transmitted from Lastpass.