404 when using add_permastruct()

Question

My wordpress site has the following URL structure defined in WP Admin > Permalinks

Custom Structure : /articles/%postname%/

For all my default posts.

I also have the following rule defined within my functions.php file:

add_action('init', 'tdd_add_rewrite_rules');

function tdd_add_rewrite_rules()
{
  global $wp_rewrite;

  $wp_rewrite->add_rewrite_tag('%price%', '([^/]+)', 'price=');
  $wp_rewrite->add_rewrite_tag('%material%', '([^/]+)', 'material=');

  $wp_rewrite->add_permastruct('prices', '/%material%/%price%', array( 'walk_dirs' => false, 'with_front' => false ));

}

I pass with_front = false so that these specific links do not contain articles/ in the slug.

Using add_permastruct() correctly now works for the urls that contain a price and material i.e

mysite.com/copper/500

But now any other post url is resulting in a 404 error. i.e

mysite.com/articles/post-name

It seems the add_permastruct() function is overwriting any url with 2 parts to the slug. No matter what they are.