Order refund amount incorrect

Description

• Precondition: An order has 2 items in Completed status.
• Steps to reproduce:

1. Create a return then create an exchange for the first item.
2. Complete the return of the first item. The refund amount is correct.
3. Create then complete the return of the second item.

• Expected: Refund box shows correct amount, or user can finish the correct amount by typing.
• Actual result: The refund amount is incorrect (Only the shipping fee appears). If you type the exact amount into the form then click Ok, you see an error.