I'm also having this problem. I'm not able to use required fields with AJAX, and if I disable AJAX the first attempt to submit the form without any of the required fields disables the submit button. The submit button does NOT become enabled after the required field is provided.
I'm sorry. It turns out that this problem is due to caching. My host, WP Engine, support helped me find the issue. Anyone who is using WP Engine, must have caching turned off for these pages to work.
Now I'm seeing that this form is not working with or without AJAX. I do not have any required fields. I'm running the newest version. The form works if I'm logged in to WordPress. The form does not work when I use a private window. Here is my code: https://pastebin.com/WsS2cAg3
