diff -u b/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
--- b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -324,9 +324,8 @@
         // Now install the module's schema if necessary.
         drupal_install_schema($module);

-        // Clear plugin manager caches and flag router to rebuild if requested.
+        // Clear plugin manager caches.
         $this->pluginCacheClearer->clearCachedDefinitions();
-        $this->routeBuilder->setRebuildNeeded();

         // Set the schema version to the number of the last update provided by
         // the module, or the minimum core schema version.
@@ -512,9 +511,8 @@
       // its statically cached list.
       drupal_static_reset('system_rebuild_module_data');

-      // Clear plugin manager caches and flag router to rebuild if requested.
+      // Clear plugin manager caches.
       $this->pluginCacheClearer->clearCachedDefinitions();
-      $this->routeBuilder->setRebuildNeeded();

       // Update the kernel to exclude the uninstalled modules.
       $this->updateKernel($module_filenames);