diff -u b/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
--- b/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -812,7 +812,7 @@
       unset($b['drupalSettings']);
     }
     // Apply the normal merge.
-    $a = array_merge($a, $b);
+    $a = array_merge_recursive($a, $b);
     if (isset($drupalSettings)) {
       // Save the custom merge for the drupalSettings.
       $a['drupalSettings'] = $drupalSettings;