title: Symfony 4 addFlash appears on network tab but not on the page
body: <p>The strange thing I've got here. I've copied everything from S4 Doc's, however it does not seem working as intended.
  Controller code:</p>
  
  <pre><code>// Adding a success type message
  $this-&gt;addFlash("success", "This is a success message");
  // Adding a warning type message
  $this-&gt;addFlash("warning", "This is a warning message");
  // Adding an error type message
  $this-&gt;addFlash("error", "This is an error message");
  // Adding a custom type message, remember the type is totally up to you !
  $this-&gt;addFlash("bat-alarm", "Gotham needs Batman");
  // 2. Retrieve manually the flashbag
  // Retrieve flashbag from the controller
  $flashbag = $this-&gt;get('session')-&gt;getFlashBag();
  // Set a flash message
  $flashbag-&gt;add("other", "This is another flash message with other type");
  // Render some twig view
  $carList = $this-&gt;getDoctrine()-&gt;getRepository(CarProject::class)-&gt;findAll();                
  return $this-&gt;render('index.html.twig', array ('carList' =&gt; $carList));
  </code></pre>
  
  <p>index.html.twig code:</p>
  
  <pre><code>&lt;div&gt;
        {% for flash_message in app.session.flashBag.get('success') %}
          &lt;div class="alert alert-success"&gt;
          {{ flash_message }}
     &lt;/div&gt;
        {% endfor %}
     &lt;/div&gt;
  </code></pre>
  
  <p>I can see that it is loaded on Chrome DevTools Network tab being the latest response on the list, however it does not appear on the actual page.
  <a href="https://i.stack.imgur.com/LMUlx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LMUlx.png" alt="enter image description here"></a>
  Any ideas?</p>