User does not appear in post_author_override drop down list

I have created this post type for the Human Resources role:

// Careers
add_action( 'init', 'create_careers' );

function create_careers() {
	register_post_type( 'careers',
		array(
				'labels' => array(
				'name' => __( 'Careers' ),
				'singular_name' => __( 'Career' ),
				'add_new' => __( 'Add New' ),
				'add_new_item' => __( 'Add New Career' ),
				'edit' => __( 'Edit' ),
				'edit_item' => __( 'Edit Career' ),
				'new_item' => __( 'New Career' ),
				'view' => __( 'View Career' ),
				'view_item' => __( 'View Career' ),
				'search_items' => __( 'Search Careers' ),
				'not_found' => __( 'No Careers found' ),
				'not_found_in_trash' => __( 'No Careers found in Trash' ),
			),
			'public' => true,
			'capabilities' => array(
					'edit_post' => 'edit_career',
					'edit_posts' => 'edit_careers',
					'edit_others_posts' => 'edit_others_careers',
					'publish_posts' => 'publish_careers',
					'read_post' => 'read_careers',
					'delete_post' => 'delete_careers',
					),
			'rewrite' => array( 'slug' => 'opportunities/careers', 'with_front' => false ),
			'supports' => array( 'title', 'editor', 'excerpt', 'author', 'revisions'),
		)
	);
}

In the Members Plugin I have assigned the following capabilities for the Human Resources role:

delete_careers
edit_careers
edit_career
edit_others_careers
edit_other_career
publish_careers
read_careers
read
upload_files

I have 2 users that have the Human Resources role – test and test2.

When I log in as a Human Resources role (user is test) I can’t select other Human Resources users when editing careers – because the Author Metabox is not present.
I can see admins and editors but not sibling Human Resource roles in the post_author_override when logged in as Admin.

For example test creates a post. Test logs out and logs in as Admin. Admin can see the post_author_override drop down list containing only one of the Human Resources Role — the one that created the post originally. I can’t change it to user test2.

Additionally I get a You are not allowed to edit this post error if logged in as Human Resources person.