diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -677,8 +677,12 @@
       // @todo Replace this when we refactor the installer to use a request-
       //   response workflow.
       if ($output instanceof Response) {
+        if (\Drupal::request()->hasSession()) {
+          \Drupal::request()->getSession()->save();
+        }
+        // Send the response.
         $output->send();
-        $output = NULL;
+        exit;
       }
       // The task is complete when we try to access the batch page and receive
       // FALSE in return, since this means we are at a URL where we are no
@@ -1547,8 +1551,6 @@
   $session = \Drupal::service('session');
   \Drupal::request()->setSession($session);
   $session->start();
-  // Ensure the session is maintained throughout the install.
-  $session->set('_drupal_installing', TRUE);
 }
 
 /**
@@ -1846,9 +1848,6 @@
   // Installation profiles are always loaded last.
   module_set_weight($profile, 1000);
 
-  // Clean up the session flag.
-  \Drupal::service('session')->remove('_drupal_installing');
-
   // Build the router once after installing all modules.
   // This would normally happen upon KernelEvents::TERMINATE, but since the
   // installer does not use an HttpKernel, that event is never triggered.