diff -u b/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
--- b/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -90,6 +90,7 @@
       $identifier = str_replace('__', '##', $identifier, $double_underscore_replacements);
     }
     $identifier = str_replace(array_keys($filter), array_values($filter), $identifier);
+    // Replace temporary placeholder '##' with '__' only if needed.
     if ($double_underscore_replacements > 0) {
       $identifier = str_replace('##', '__', $identifier);
     }