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
@@ -209,9 +209,12 @@
       return '';
     }
 
-    // Render only the children if the #render_children property is set.
+    // Render only the children if the internal #render_children property is
+    // set.
+    // @see \Drupal\Core\Theme\ThemeManager::render().
     if (isset($elements['#render_children'])) {
-      // A non-empty #children property takes precedence.
+      // A non-empty #children property takes precedence. This happens only if
+      // it has been manually set into the render array.
       if (!empty($elements['#children'])) {
         $children = ['#children'];
       }
@@ -223,9 +226,10 @@
         return '';
       }
 
-      // It is okay to modify the original elements for this, because this will
-      // be called from a twig template, where the variable is passed by value.
-      $elements = array_intersect_key($elements, array_flip($children));
+      // Avoid making this change for the state because it could cause
+      // unexpected behaviour with other templating engines than Twig.
+      $new_elements = array_intersect_key($elements, array_flip($children));
+      $elements = &$new_elements;
     }
 
     if (!isset($elements['#access']) && isset($elements['#access_callback'])) {