Hello Giorgio,
In your case your issue is probably coming from the fact that one of your plugins which use the "hikashop_product_price_for_quantity_in_order" trigger is affecting your subtotal price without tax.
I directly tested it by checking the code, and the issue is coming from these lines on the file "/dev/administrator/components/com_hikashop/classes/order.php" :
if(function_exists('hikashop_product_price_for_quantity_in_order')) {
hikashop_product_price_for_quantity_in_order($order->products[$k]);
} else {
$order->products[$k]->order_product_total_price_no_vat = $product->order_product_price*$product->order_product_quantity;
$order->products[$k]->order_product_total_price = ($product->order_product_price+$product->order_product_tax)*$product->order_product_quantity;
}
But note that before finding from which plugin your issue is coming, a solution can be to replace these lines by :
/*if(function_exists('hikashop_product_price_for_quantity_in_order')) {
hikashop_product_price_for_quantity_in_order($order->products[$k]);
} else {*/
$order->products[$k]->order_product_total_price_no_vat = $product->order_product_price*$product->order_product_quantity;
$order->products[$k]->order_product_total_price = ($product->order_product_price+$product->order_product_tax)*$product->order_product_quantity;
//}