diff --git a/core/lib/Drupal/Core/Session/SessionManager.php b/core/lib/Drupal/Core/Session/SessionManager.php
index 95495cfd03..888145d8b0 100644
--- a/core/lib/Drupal/Core/Session/SessionManager.php
+++ b/core/lib/Drupal/Core/Session/SessionManager.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Database\Connection;
 use Drupal\Core\DependencyInjection\DependencySerializationTrait;
 use Symfony\Component\HttpFoundation\RequestStack;
+use Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface;
 use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;
 
 /**
@@ -312,15 +313,20 @@ protected function getSessionDataMask() {
     $mask = array_fill_keys(array_keys($_SESSION), TRUE);
 
     // Ignore the metadata bag, it does not contain any user data.
-    $mask[$this->metadataBag->getStorageKey()] = FALSE;
 
     // Ignore attribute bags when they do not contain any data.
+    $data = [];
     foreach ($this->bags as $bag) {
       $key = $bag->getStorageKey();
-      $mask[$key] = !empty($_SESSION[$key]);
+      if (!empty($_SESSION[$key])) {
+        $data[$key] = $_SESSION[$key];
+      }
+      if ($bag instanceof FlashBagInterface && ($messages = $bag->peekAll())) {
+        $data[$key] = $messages;
+      }
     }
 
-    return array_intersect_key($mask, $_SESSION);
+    return $data;
   }
 
   /**