Total manual discount incorrect when adding item to cart if quantity > CM max quantity setting

Description

Steps to reproduce:

1. Go to Commerce Manager.
2. Create a new cart.
3. Select a new line item to add to cart (for example Tank Maxi Dress with price $12, max quantity setting is 100).
4. In New Line Item dialog, set the quantity too 1000. Manual discount is off 1$ value base.
5. Click OK
6. The item quantity is updated to max quantity setting 100.

Expect:
Total discount is calculated based on actual quantity, $100.

Actual:
Total discount is calculated based on input quantity, $1000.

Note: This bug also happens with purchase orders.
Note: Issue was fixed in 10.3.0 and reoccurs in 12.13.0.