Wrong total discount on returns tab

Description

Step to reproduce

1. Create a new order promotion.
2. Add 1 item to a cart which satisfies the promotion condition and complete the order.
3. Go to Commerce Manager > create a new return, select 1 item to return > The total discount displays correctly.
4. Go to the Return tabs and view the return detail.

Expected: The total discount displays the correct discount amount for returned items
Actual: The total discount displays the amount of all items in the order.

Note: This issue occurs in the old and new promotion systems.