So, in the codex and register_post_type, I've come across the code tha allows you to put custom post types in admin menu submenus using the following:
'show_in_menu' => 'edit.php?post_type=parent_post_type_slug'
However, when I select "Create New" on this page however so the URL is now something like:
The parent_post_type admin menu IS NOT highlighted. Although definitely not a critical issue, it's still an incorrect implementation issue when a user specifies a value for the 'show_in_menu' argument when registering a new custom post type.
Is there any work around? Can someone confirm that this is unexpected/desirable behavior?