When a parent menu item AND its child menu item are marked as hidden
the child's visibility is inverted here and ends up displayed without its parent (plus triggers a Warning: Undefined array key 19 in /app/wp-includes/nav-menu-template.php on line 211 notice)
source: https://wordpress.org/support/topic/hidden-sub-menus-are-displayed-when-main-menu-item-is-hidden/#post-16341589
When a parent menu item AND its child menu item are marked as hidden
the child's visibility is inverted here and ends up displayed without its parent (plus triggers a
Warning: Undefined array key 19 in /app/wp-includes/nav-menu-template.php on line 211
notice)