category_display parameter

I can add tags and categories to pages using the following code in my theme’s functions.php file:

function add_taxonomies_to_pages() {
 register_taxonomy_for_object_type( 'post_tag', 'page' );
 register_taxonomy_for_object_type( 'category', 'page' );
 }
add_action( 'init', 'add_taxonomies_to_pages' );

I can then use [display-posts category="test" post_type="page"] to list all pages in the test category.

However, I cannot add [display-posts category_display="true"] to a page in the test category and get it to display anything (unless I also add a post to the test category.)

Any help would be much appreciated.