Cart items have the same "changed" date

I wanted to do some sorting according to "changed" timestamp in the "uc_cart_products" table.

There is a thread Illogical ordering of cart contents, but that ordering won't work.

To reproduce:

1. add item to the cart
2. add another item to the cart

check the "uc_cart_products" table, items have different "changed" timestamp

go to /cart and update one of the items with new amounts

check the "uc_cart_products" table, both items have the same "changed" timestamp