This scenario is quite specific and happens only in version 5 of the checkout (which doesn't have the omnichannel features). It happens in the carts where the customer chooses to pay with a Gift Card and 2 cards. The amount to be paid for one of these cards is changed shortly after being entered by the customer.

1. Create a cart and go to the payment step;
2. Fill in/select the Gift Card;
3. Choose to pay with 2 cards;
4. Enter the amount to be paid on the first card;
5. Notice that the value has been changed to a smaller one.

We currently have no workaround for this problem.