I am currently suffering from a similar issue again and I'm not sure how to remedy this.
I've been holding off using the latest Pro Sites versions because they would constantly cause issues to my signup page (https://mydestinyexpress.com/pro/). Recently the sign-up page was no longer working so I upgraded to the latest Pro-Sites version 18.104.22.168, (it was previously 3.2.something).
The signup page consistently redirects to "https://mydestinyexpress.com/pro/?action=new_blog" when a visitor goes to the link "https://mydestinyexpress.com/pro/". How can I solve this?
I posted this on another forum page (https://premium.wpmudev.org/forums/topic/pro-sites-new-site-registration#post-990288) but reposted here just in case I wasn't supposed to.