Hi,
Mmm well it's not really a bug. This allows to you update the quantity to unlimited as -1 means unlimited in HikaShop.
The problem is that you can't decrease the quantity with the 'Update the product quantity instead of replacing it' option turned on.
If you want to change that, you can replace the line:
if(JRequest::getInt('update_product_quantity','0') && $field=='product_quantity' && $product->product_quantity != -1){
to:
if(JRequest::getInt('update_product_quantity','0') && $field=='product_quantity'){
in the file administrator/components/com_hikashop/helpers/import.php