No content output

I am trying to output some code on a custom route within my default template. I am using the following code…

add_action( 'wp_router_generate_routes', 'sf_add_routes', 20 );

function sf_add_routes( WP_Router $router ) {
    $route_args = array(
        'path' => '^dashboard',
        'query_vars' => array( ),
        'page_callback' => 'sf_dashboard_route_callback',
        'page_arguments' => array( ),
        'access_callback' => true,
        'title' => __( 'Dashboard' ),
        //'template' => false
    );

    $router->add_route( 'dashboard', $route_args );
}

function sf_dashboard_route_callback( ) {
    echo "This will be the dashboard";
}

It’s in a plugin and the route works but the echo isn’t output. If I set template to false or add exit into the callback the string is output without the rest of the page rendering.

The sample route included in the plugin also behaves in the same way.

I have tried copying in the default page.php from twentyfourteen and twentytwelve but all act in the same way.

Any ideas?

https://wordpress.org/plugins/wp-router/