Don't render subrequest as a full page, just pass them through normal drupal_render().

core/lib/Drupal/Core/EventSubscriber/ViewSubscriber.php

@@ -63,7 +63,8 @@ public function onView(GetResponseForControllerResultEvent $event) {
       }
     }
     else {
-      $event->setResponse($this->onHtml($event));
+      $page_callback_result = $event->getControllerResult();
+      $event->setResponse(new Response(drupal_render($page_callback_result)));
     }
   }