Menu Order Not Maintained

You’re providing the argument menu-order to wp_insert_post() rather than the correct menu_order, and as such, the order of the duplicated menu is not correct.

I currently have the following workaround in place:

add_filter('wp_insert_post_data', function ($data, $postarr) {
    if ($data['post_type'] === 'nav_menu_item' && isset($postarr['menu-order'])) {
        $data['menu_order'] = (int) $postarr['menu-order'];
    }

    return $data;
}, 10, 2);