export custom_checkout_fields

Hello,

I use custom checkout fields ( 3 fields) and its not possible to find them when I want to choose field for export..
For example for “nom_majeur” :

woocommerce_form_field('nom_majeur', array(
        'type' => 'text',
        'class' => array('form-row-first'),
        'label'       => __('Je soussigné(e)') . ' <span class="tooltip-icon-mineure" title="Veuillez entrer votre nom complet.">ℹ️</span>',
        'required' => false,
    ), $checkout->get_value('nom_majeur'));

function custom_checkout_field_update_order_meta($order_id) {
    if (!empty($_POST['nom_majeur'])) {
        update_post_meta($order_id, 'nom_majeur', sanitize_text_field($_POST['nom_majeur']));
    }
}

Can someone help me ? Thanks