Pro Sites suddenly redirects wp-signup.php to /pro-site?action=new_blog

I don’t know how long this has been happening, but as it says in the subject line, people were able to go to the signup page and sign up for a free site on my network. It’s now started redirecting them to the Pro Site membership page. I’ve had to disable the plugin, which is a major inconvenience for some users. Why did this suddenly start happening? I haven’t installed any other plugins recently or made any changes whatsoever to the site structure. Access has been granted.