Hierarchical taxonomy as custom post type path – URL rewrite issues

I have a custom post type (collections) with:
'rewrite' => array('slug' => 'collections/%collection_cat%', 'with_front' => false, 'feeds' => false)

and a custom taxonomy (collections_cat) with:
'rewrite' => array('slug' => 'collections', 'with_front' => false, 'hierarchical' => true)

I’m filtering the rewrite_rules_array with:

$rules['collections/(.+)/?$']           = 'index.php?collection_cat=$matches[1]';
$rules['collections/(.+)/(.+)/(.+)/?$'] = 'index.php?collections=$matches[3]';

On front-end /collections/cat/ and /collections/cat/subcat/ work fine.

/collections/cat/subcat/postname throws 404. Looking at $query, the postname isn’t recognized separately as the third match for the rewrite rule. So instead of ?collections=postname, ?collection_cat=cat/subcat/postname is queried.

What is going on?

I’ll appreciate any help I can get.

Thanks