Navigation does not render correctly when using `level: 2`

[Flauschbaellchen]

I'm splitting my navigation into two seperate divs. One at the top and one as a sidebar.
simple-navigation provides a :level setting to archive this functionality (docs)

Using the Bootstrap3 renderer, the second menu is hidden (display: none;) as it is rendered like a dropdown menu.

View:

= render_navigation expand_all: true, level: 2

Navigation settings:

navigation.renderer = SimpleNavigationBootstrap::Bootstrap3
navigation.items do |primary|
    primary.item :main, 'Main', main_url do |sub_nav|
      sub_nav.item :sub, 'Sub', sub_url
    end
end

Level 2 should be rendered like level 1, level 3 like level 2 and so on.