我的结局是：

// Register Custom Taxonomy
function fixed_tags_taxonomy() {

    $labels = array(
        \'name\' => _x( \'Fixed Tags\', \'Taxonomy General Name\', \'text_domain\' ),
        \'singular_name\' => _x( \'Fixed Tag\', \'Taxonomy Singular Name\', \'text_domain\' ),
    );

    $args = array(
        \'labels\' => $labels,
        \'hierarchical\'=>true,
    );

    register_taxonomy( \'fixed-tags\', array( \'portfolio\' ), $args );
}
add_action( \'init\', __NAMESPACE__ . \'\\\\\' . \'fixed_tags_taxonomy\', 2 );

// rewrite url portfolio post type
// we want something in this format -> portfolio/{fixed_tag_name}  
// remember to visit wp-admin/options-permalink.php and click "save changes"
add_action(\'init\', function() {

    add_rewrite_tag( \'%portfolio%\', \'([^&]+)\' );
    add_rewrite_rule( \'^portfolio/([^/]*)/?\', \'index.php?portfolio=$matches[1]&fixed_tags=$matches[1]\',\'top\' );

    // add_rewrite_tag( \'%portfolio%\' );
    add_rewrite_rule( \'^portfolio$\', \'index.php?pagename=portfolio\',\'top\' );

}, 10, 0);

// we need to register our query variable to wordpress 
// will pass it\'s value back to us when we call get_query_var(\'fixed_tags\'))
add_filter( \'query_vars\', function( $query_vars )
{
    $query_vars[] = \'fixed_tags\';
    return $query_vars;
});

add_filter(\'template_redirect\', function () 
{
    global $wp_query;

    $fixed_tag = get_fixed_tag(); 

    if ($fixed_tag) {
        status_header( 200 );
        $wp_query->is_404=false;

        $wp_query->set(\'post_type\', \'portfolio\');
        $wp_query->set(\'tax_query\', [
            \'relation\' => \'AND\',
                array(
                \'taxonomy\' => \'fixed-tags\',
                \'field\'    => \'slug\',
                \'terms\'    => array( $fixed_tag ),
            )
        ]);       
    }
});

add_filter( \'template_include\', function ( $template ) 
{
    $fixed_tag = get_fixed_tag();

    if($fixed_tag){
        return locate_template( [\'page-portfolio-flex-layouts.php\' ] );
    }

    return $template;
});

function get_fixed_tag()
{
    $fixed_tag = sanitize_title(get_query_var(\'fixed_tags\'));

    // we check if term is fixed tags
    // we tell wordpress to handle as 200 is found
    $term = get_term_by(\'slug\', $fixed_tag, \'fixed-tags\'); 

    return $term ? $fixed_tag : null;
}