Quantity dropdown issue on updating cart

Hi.
We have set in our shop the quantities by drop down instead of box. Everything is fine but when we go to the “Cart”, change the quantity of any product by drop down and update cart only the first time is changed. If you try to change it again and update cart the quantity hasn´t changed, it has the value of the first change.

For example:

Product 1 -> Qty = “3” and add to cart

Go to cart and change the Qty = “2” and update cart.

The value is correct but if I try to change it again Qty = “1”, for example, afte updating cart it continues showing “2”

This happens when the quantity is as dropdown not as box.

Any idea what can be happenning?

Best regards.