item.below is empty in menu.html.twig on top level menu

API page: https://api.drupal.org/api/drupal/core!themes!stable!templates!navigatio...

I try to render the main navigation with all it's child menu items as a menu block in my navigation region. I configured the block under admin/structure/block/manage/THEMENAME as 'Initial menu level' 1 and 'Maximum number of menu levels to display' 2.

The menu has the following hierarchy.

-- menuitem
-- menuitem
-- menuitem (parent)
  -- menuitem (child)

My main navigation consists of 2 levels which get rendered when I navigate to the menu item which has a child (e.g. parent). But I try to build a menu which renders all child elements on every level (also on toplevel / front, not only when I already navigated to the parent page which has a child). E.g. when you try to build a 2-level-hover-menu.

But the variable item.below in menu.html.twig is always empty on toplevel elements respectively if it is not the parent page ... e.g. front.

Any suggestions how to tackle this?