Prestashop 1.6 - Order discount not recalculated after order edited

[matamata365]

All my orders have 100% discount coupon applied to the total price of each order. The problem is, when I edit the order (in admin panel) by updating some product's quantity and hit SAVE, then the discount is not being recalculated in relation to the total price which had changed.

I can understand some basic PHP. Can you tell me which files should I update?

 

Thanks 

Edited July 27, 2020 by matamata365 (see edit history)