Middleware not working when created route from routeserviceprovider in laravel

Create the Route file from the Route Service Provide and assign the middleware "admin.auth" and this middleware working in the web.php giving the basic information of the admin user but from the custom.php it is returning auth false. How can admin.auth will work from route service provider

 protected function mapWebRoutes2()
    {
        Route::group([
            'namespace' => $this->namespace,
            'prefix' => 'custom',
            'middleware' => 'admin.auth'
        ], function ($router) {

            require base_path('routes/custom.php');
        });
    }

via Chebli Mohamed