Howdy,
Kraft from Automattic/Jetpack:
The Contact Form on submit reloads the current page with items in the POST (using the PHP superglobal). Not being sure how ACF handles this exactly, if the POST isn’t able to be passed through the form and/or the code isn’t loaded until late, etc, I could see this short-circuiting the process.
We look for the POST here: https://github.com/Automattic/jetpack/blob/master/modules/contact-form/grunion-contact-form.php#L111-121
and then process that via
https://github.com/Automattic/jetpack/blob/master/modules/contact-form/grunion-contact-form.php#L141-210
Thanks!