I think the issue is in Jigoshop Settings / Catalog & Pricing / Sorting via Product Post Order. To display my Products ID for check, I added this code to loop-shop.php:
$my_products_id = array();
while ( have_posts() ) : the_post();
array_push($my_products_id, $post->ID);
...
endwhile;
and this to the head of products listing area (in the same file) :
...
$content .= '<div><b>' . implode (', ', $my_products_id) . '</b></div>
...
I noticed the products are just listed randomly (not ASC nor DESC) and some POST ID are repeated in the next page. When I change Sort products in catalog by to Post Date, everything is just fine. Product ID are correctly sorted ASC or DESC, depending on which order I choose.
I guess sorting via Product Post Order means Post ID, which must be like sorting via Post Date (the most recent product is the one with the highest ID or the most recent date). So sorting via Product Post Order must have the same effect like sorting via Post Date for the same order (ASC or DESC). But for some reason, sorting via Product Post Order does not work.