diff -u b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
--- b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
+++ b/core/lib/Drupal/Core/Authentication/AuthenticationManager.php
@@ -65,7 +65,8 @@
    *   The providers priority.
    */
   public function addProvider(AuthenticationProviderInterface $provider, $id, $priority = 0) {
-    $id = substr($id, strlen('authentication.'));
+    // Remove the 'authentication.' prefix from the provider ID.
+    $id = substr($id, 15);
 
     $this->providers[$id] = $provider;
     $this->providerOrders[$priority][$id] = $provider;