OK, try adding this PHP Snippet:
add_filter( 'wp_nav_menu_objects', 'add_menu_parent_class' );
function add_menu_parent_class( $items ) {
$parents = array();
foreach ( $items as $item ) {
if ( in_array('current-page-ancestor', $item->classes) ) {
$parents[] = $item->menu_item_parent;
}
}
foreach ( $items as $item ) {
if ( in_array( $item->ID, $parents ) ) {
$item->classes[] = 'current-menu-ancestor';
}
}
return $items;
}
If that works it should add the current-menu-ancestor class to the top level item.
Let me know 🙂