diff -u b/core/includes/update.inc b/core/includes/update.inc
--- b/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -103,7 +103,9 @@
   $settings['cache']['default'] = 'cache.backend.memory';
   new Settings($settings);
 
-  // Enable UpdateBundle service overrides.
+  // Enable UpdateBundle service overrides. While the container_bundles array
+  // does not need a key, let's use so it can be removed once the upgrade are
+  // finished. @see update_flush_all_caches()
   $GLOBALS['conf']['container_bundles']['UpdateBundle'] = 'Drupal\Core\DependencyInjection\UpdateBundle';
 
   // Check whether settings.php needs to be rewritten.