Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/themes/stable/templates/block/block--system-menu-block.html.twig

@@ -38,7 +38,7 @@
     {% set title_attributes = title_attributes.addClass('visually-hidden') %}
   {% endif %}
   {{ title_prefix }}
-  <h2{{ title_attributes }}>{{ configuration.label }}</h2>
+  <h2{{ title_attributes.setAttribute('id', heading_id) }}>{{ configuration.label }}</h2>
   {{ title_suffix }}
 
   {# Menu. #}