I don't think it is just the promotion thing but also the 'fullfilled by Lazada' that keeps you from being able to cancel an item.
Not everthing sold on Lazada is 'fullfilled by Lazada'.
But yes it is getting harder to cancel orders on Lazada. Lots of sellers automatically put the packed label on the order as soon as they receive it which also keeps you from being able to cancel the order. Even though it obvious on some orders that they never actually packed it yet, let alone shipped it.