javascript – “Load failed” on fetch request, however solely within the Instagram app

I take advantage of a type on my WordPress web site. The info is shipped asyncronously to wp-ajax.php. This works high-quality in all browsers, however within the Instagram app browser in iOS I get the error “Load failed”. The fascinating factor is that the request nonetheless arrived on the server and was processed accurately. What may very well be the rationale for this?

<script>
    doc.querySelector('.gm_courseform').addEventListener('submit', perform(occasion) {
        occasion.preventDefault();

        const type = this;
        type.classList.add('sending');

        const submitButton = doc.getElementById('submit_button');
        const originalButtonText = submitButton.textContent;
        submitButton.disabled = true;
        submitButton.textContent = "Wird geladen...";
        doc.querySelectorAll(".form-message").forEach(el => el.take away());
        const formData = new FormData(this);
        formData.append('motion', 'create_booking');

        fetch("<?php echo admin_url('admin-ajax.php'); ?>", {
                methodology: "POST",
                headers: {
                    "Cache-Management": "no-cache, no-store, must-revalidate",
                    "Pragma": "no-cache",
                    "Expires": "0"
                },
                physique: formData,
            })
            .then(response => response.json())
            .then(information => {
              // [...]
            })
            .catch(error => {
             // [...]
    });
</script>