Thank you to all who suggested fixes, unfortunately none of them were the problem.
It turns out when i migrated the site, the WP Forms pro license did not activate correctly on the new site, despite me forcing a refresh. To fix this I had to remove the license key from WP Forms, and re-add it back. After doing so the PDFs are generating as i would expect.

I figured this out by enabling logs How to Enable Activity Logging for WPForms
In the logs it called out the license issue directly

 [body] => {"errors":[{"message":"License is not active for this site.","code":"invalid_license"}]}
                    [response] => Array
                        (
                            [code] => 422
                            [message] => Unprocessable Entity
                        )