Foundation handles the HTML population with their “reveal” module.
See comment above.
Here is the source for the trigger that I used in the code above.
The AJAX callback and DOM creation begins on Line 144.
My AJAX function is actually the parameter ajax_settings that is being used on Line 144 and Line 158.
As a note – I can confirm that the popup does open and revel. I can also confirm that the HTML of the form from my WordPress AJAX callback does indeed fill the popup container. The popup container is the very first child in the <body> tag. And the acf_head() is at the very top of my HTML and is placed via the plugins_loaded hook with a priority of 1
