Enabled subtree links of a disabled parent link are added to the menu tree at the wrong depth.

Problem/Motivation

Having the following menu tree:

- item 1 (disabled)
-- item 1a
-- item 2a
- item 2
-- item 2a 
- item 3

Then fetch the full menu tree with only enabled menu items:

$parameters = new MenuTreeParameters();
$parameters->onlyEnabledLinks();

$tree = \Drupal::service('menu.link_tree')->load($menu_name, $parameters);

I expect to get the following tree:

- item 2
-- item 2a 
- item 3

Instead I get:

- item 1a
-- item 2a
- item 2
-- item 2a 
- item 3

Proposed resolution

Don't add enabled subtree items to the tree when the parent item is disabled and the menu tree is build with the "onlyEnabledLinks" parameter set.