If you export a menu link, which is marked as expanded, and with other feature add some children it is not showed expanded because in the feature 'has_children' is 0 (false) and not 1.

$menu_links['main-menu:listings'] = array(
    'menu_name' => 'main-menu',
    'link_path' => 'listings',
    'router_path' => 'listings',
    'link_title' => 'Listings',
    'options' => array(
      'attributes' => array(
        'title' => 'Listings frontpage',
      ),
    ),
    'module' => 'system',
    'hidden' => '0',
    'external' => '0',
    'has_children' => '0',
    'expanded' => '1',
    'weight' => '7',
  );

However, if you go to menu link edit page and save it drupal realize that it has children and change this value but now features shows it overriden when you didn't change anything.
I think features must not pay attention to this value and leaves Drupal works with it.