Admin menu hilighting wrong item for CPT

Check:

add_submenu_page(
    'edit.php?post_type=landingpage', 
    'Form Settings',
    'Form Settings',
    'manage_options', 
    // This is the menu slug, you can not pass the arguments,
    // if you want to pass arguments then add only one sub menu page and
    // links with arguments, also display content with conditions of passed arguments.
    'wp_cpt_plugin_page-form-settings',
    'my_callback'
);