I had the same problem (using v1.75) - when updating an order, entries belonging to different orders would disappear from the table ORDERS_PRODUCTS. I think the culprits are lines 323:
$Query = "delete from " . TABLE_ORDERS_PRODUCTS . " where orders_products_id = '$orders_products_id';";
and 332:
$Query = "delete from " . TABLE_ORDERS_PRODUCTS_ATTRIBUTES . " where orders_products_id = '$orders_products_id';";
I changed them to
$Query = "delete from " . TABLE_ORDERS_PRODUCTS . " where orders_id = '" . (int)$oID . "' and orders_products_id = '$orders_products_id';";
and
$Query = "delete from " . TABLE_ORDERS_PRODUCTS_ATTRIBUTES . " where orders_id = '" . (int)$oID . "' and orders_products_id = '$orders_products_id';";
So far it seems to work fine.
Giirov