The sign up source is appearing and disappearing

Hi there,

I’ve used a hidden field for my sign up source. For this form only ( there are 4 sign up forms on the site) it will feed through the signup source and then the signup source just disappears. I chatted with Mailchimp about this. They say WP is making a put call to place the signup source and then doing a second put call and removing the signup source. How do I fix this?

The page I need help with: [log in to see the link]