add_filter('get_terms_args', function ($args, $taxonomies) {

    if (is_admin()) return $args;

    if (in_array('project_category', (array) $taxonomies, true)) {
        $args['meta_key'] = 'order';
        $args['orderby'] = 'meta_value_num';
        $args['order']   = 'ASC';
    }

    return $args;
}, 10, 2);
