(In this example I've just added code to the AcmeDemoBundle to make it easier to reproduce):

Twig code added to AcmeDemoShopBundle:Demo:index.html.twig:
{% render 'AcmeDemoBundle:Demo:hello2' with {'_format' : 'otherformat'} %}

Controller code added to Acme\DemoBundle\Controller\DemoController:
/* @route("/hello2.{_format}", name="_demo_hello2", defaults={"_format" = "defaultformat"})

• @template()
  */
  public function hello2Action($_format) {
  return array('_format' => $_format);
  }

When the Twig template calls the render function the _format parameter is ignored and the controller always see "html" instead 'otherformat' (specified in the render function).