POST parameters not accessible with AJAX post request

Hello everyone!
I am currently facing an issue which I have no idea how to solve... this is quite infuriating.
I have a contact form that sends its content to a controller with the fetch javascript function and then I try to access the ajax body parameters from within the controller with $this->request->getPost('email') but to no avail.
What works, however, is using: $this->request->getJsonVar('email');

What is weird is that I have the CSRF protection enabled and that the system manages to detect the csrf parameter.

Here is my HTML form :

Code:

<form action="<?php echo site_url('send-mail'); ?>" method="post" onsubmit="return post_contact_form(this);">
    <?php
        $error = session()->getFlashdata('error');
        if($error):
        ?>
        <div class="alert alert-danger">
            <?php echo $error; ?>
        </div>
        <?php
        endif;
    ?>
    <div class="mb-2">
        <label for="contact-experience-email" class="form-label">Votre mail</label>
        <input placeholder="Votre adresse mail" name="contact-form-email" class="form-control" type="email" />
    </div>
    <div class="mb-2">
        <label for="contact-experience-phone-number" class="form-label">Votre téléphone</label>
        <input placeholder="Votre numéro de téléphone" name="contact-form-phone-number" class="form-control" type="text" />
    </div>
    <div class="mb-2">
        <label for="contact-experience-details" class="form-label">Des détails supplémentaires ?</label>
        <textarea placeholder="Quel est votre besoin ?" name="contact-form-details" class="form-control" id="" cols="30" rows="5"></textarea>
    </div>
    <button type="submit" class="cta_1">Cliquer pour valider ma demande</button>
    <?= csrf_field() ?>
</form>

Here is my javascript code :


function post_contact_form(form){
    let email_input = form.querySelector("[name=contact-form-email]");
    let phone_input = form.querySelector("[name=contact-form-phone-number]");
    let details_input = form.querySelector("[name=contact-form-details]");
    let csrf = form.querySelector("[name=jeton_csrf]");
    let post_url = form.getAttribute("action");

    // let my_form_data = new FormData(form);
    let my_form_data = {
        "email": email_input.value,
        "phone_number": phone_input.value,
        "details": details_input.value,
        "jeton_csrf": csrf.value
    };

    fetch(post_url, {
        method: "POST",
        headers: {
            "Content-Type": "application/json",
            "X-Requested-With": "XMLHttpRequest"
        },
        body: JSON.stringify(my_form_data)
    })


Here is my route :

$routes->post('send-mail', 'MailController::sendMail');

Here is my controller

class MailController extends \App\Controllers\BaseController
{
    public function sendMail(){
        if (strtolower($this->request->getMethod()) !== 'post') {
            return $this->response->setStatusCode(405)->setBody('Method Not Allowed');
        }

        if(!$this->request->isAJAX()){
            return $this->response->setStatusCode(405)->setBody('Method Not Allowed');
        }
//        $post_data = json_decode($this->request->getBody(), true);
        $v_email = $this->request->getJsonVar('email');
        $v_phone_number = $this->request->getJsonVar('phone_number');
        $v_details = $this->request->getJsonVar('details');
        $my_post_data = [$v_email, $v_phone_number, $v_details];

        var_dump($this->request->getPost('email')); // null

//        return json_encode($post_data);
        return json_encode($my_post_data); // ["[email protected]","+330102030405","Lorem"]
    }
}

You post as JSON:

Code:

    fetch(post_url, {
        method: "POST",
        headers: {
            "Content-Type": "application/json",
            "X-Requested-With": "XMLHttpRequest"
        },
        body: JSON.stringify(my_form_data)
    })

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