diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1203,7 +1203,19 @@
   $render_element_type = isset($info['render element']) ? $info['render element'] : NULL;
 
   $themeManager = \Drupal::service('theme.manager');
-  $variables += $themeManager->getDefaultTemplateVariables($render_element_type, $variables);
+  $variables += $themeManager->getDefaultTemplateVariables($render_element_type);
+
+  // Merge element #attributes into $default_variables['attributes'] if they
+  // exist in render_element.
+  // This code is dublicated in ThemeManager::render(). Moving it to
+  // ThemeManager would make the ThemeManagerInterface messy.
+  if (isset($variables['#attributes'])) {
+    $variables['attributes'] = NestedArray::mergeDeep(
+      $variables['attributes'],
+      $variables['#attributes']
+    );
+  }
+
 }
 
 /**
diff -u b/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
--- b/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -285,13 +285,14 @@
 
     // Set default variables before preprocess hooks.
     $variables += $this->getDefaultTemplateVariables();
+    $render_element_type = isset($info['render element']) ? $info['render element'] : NULL;
 
     // Merge element #attributes into $default_variables['attributes'] if they
     // exist in render_element.
-    if (isset($element['#attributes'])) {
+    if (isset($variables[$render_element_type]['#attributes'])) {
       $variables['attributes'] = NestedArray::mergeDeep(
         $variables['attributes'],
-        $element['#attributes']
+        $variables[$render_element_type]['#attributes']
       );
     }