diff --git b/core/modules/rdf/rdf.module a/core/modules/rdf/rdf.module
index ba3c00e..86aa0a0 100644
--- b/core/modules/rdf/rdf.module
+++ a/core/modules/rdf/rdf.module
@@ -440,8 +440,8 @@ function rdf_preprocess_comment(&$variables) {
   // Adds RDFa markup for the relation between the comment and its author.
   $author_mapping = $mapping->getPreparedFieldMapping('uid');
   if (!empty($author_mapping)) {
-    // Wraps the 'author' and 'submitted' variables which are both available
-    // in comment.html.twig.
+    // Wraps the 'author' and 'submitted' variables which are both available in
+    // comment.html.twig.
     foreach (['author', 'submitted'] as $variable) {
       $variables[$variable] = [
         '#type' => 'html_tag',
@@ -462,11 +462,15 @@ function rdf_preprocess_comment(&$variables) {
       '#theme' => 'rdf_metadata',
       '#metadata' => array($date_attributes),
     );
-    $created_metadata_markup = drupal_render($rdf_metadata);
     // Appends the markup to the created variable and the submitted variable
     // which are both available in comment.html.twig.
-    $variables['created'] = SafeMarkup::format('@created@metadata_markup', ['@created' => $variables['created'], '@metadata_markup' => $created_metadata_markup]);
-    $variables['submitted'] = SafeMarkup::format('@submitted@metadata_markup' , ['@submitted' => $variables['submitted'], '@metadata_markup' => $created_metadata_markup]);
+    foreach (['created', 'submitted'] as $variable) {
+      $variables[$variable] = [
+        // Ensure the original variable is represented as a render array.
+        !is_array($variables[$variable]) ? ['#markup' => $variables[$variable]] : $variables[$variable],
+        $rdf_metadata,
+      ];
+    }
   }
   $title_mapping = $mapping->getPreparedFieldMapping('subject');
   if (!empty($title_mapping)) {