What is different about using admin-ajax.php in an endpoint and just submitting a form from the front end?

Question

I have created an endpoint with the following php code. The code is currently allowing me to receive the correct pdf but none of the information from the $page is populated in the pdf. The $page is a page that lists out results of a user using shortcodes I have created that require the user id as input. The page gets the current user id required via a GET query parameter (user=###). If I send the form from the page itself it works perfectly fine, but when I attempt to replicate the submission in this endpoint the pdf is not populated with the user data.

add_filter("wcra_postresults_callback" , "wcra_postresults_callback_handler");
function wcra_postresults_callback_handler($request){
    $user_id = $request['StudentID'];
    $project = strtolower($request['Project']);
    $page = "https://mydomain.com.au/view-results-".$project;
    $student = get_userdata($user_id);
    $student_email = $student->user_email;
    $unitcomplete = url_to_postid($page);
    $pagetitle = get_the_title($unitcomplete);
    $completereferrer = $page."/?user=".$user_id;
    $formid = 0;
    
    $requestorigin = $_SERVER['HTTP_ORIGIN'];
    if ($requestorigin === "myapp") {
        switch($project):
            case 'b':
                $formid = "b7cd306";
            break;
        endswitch;
        
        // Form data (replace with actual field names and values)
        $formData = array(
            'post_id' => $unitcomplete,
            'form_id' => $formid,
            'referer_title' => $pagetitle,
            'queried_id' => $unitcomplete,
            'form_fields[emaildestination]' => $student_email,
            'action' => 'elementor_pro_forms_send_form',
            'referrer' => $completereferrer
        );

        // Initialize cURL
        $ch = curl_init();

        // Set request URL
        curl_setopt($ch, CURLOPT_URL, "https://mydomain.com.au/wp-admin/admin-ajax.php");

        // Set request method to POST
        curl_setopt($ch, CURLOPT_POST, 1);

        // Set POST data
        curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($formData));

        // Dump current data for debug
        echo json_encode($formData);

        // Capture response
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

        // Execute request
        $response = curl_exec($ch);

        // Dump form response for debug
        echo $response;

        // Close cURL handle
        curl_close($ch);

        echo 'Results sent!';
    }
}

Answer 1

Unfortunately the answer to this question is simply that you cannot do this. curl will never resolve the shortcodes of your page before you submit the form.