Basically, I have a function which should add custom fields depending on a cart quantity.

But it wasn’t working. I started debugging and this is my function and it doesn’t save posted custom field and I can’t figure out why.

add_action('woocommerce_after_checkout_billing_form', 'custom_checkout_field');
function custom_checkout_field($checkout){
    echo '<div><h4>Passenger</h4>';
        woocommerce_form_field('passenger_birthdate_2', array(
            'type'        => 'text',
            'label'       => 'Date of birth',
            'required'    => true,
        ),
        $checkout->get_value('passenger_birthdate_2'));
    echo '</div>';
}

add_action('woocommerce_checkout_update_order_meta', 'custom_checkout_field_update_order_meta');
function custom_checkout_field_update_order_meta( $order_id ) {
    if ($_POST['passenger_birthdate_2']) update_post_meta($order_id, 'passenger_birthdate_2', sanitize_text_field($_POST['passenger_birthdate_2']));
}

Am I missing something?