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
@@ -2046,6 +2046,8 @@
   \Drupal::service('config.manager')->createSnapshot($active, $snapshot);
 
   // Load current user and perform final login tasks.
+  // This has to be done after drupal_flush_all_caches()
+  // to avoid session regeneration.
   $account = user_load(1);
   user_login_finalize($account);