Ideas

As of today, the FreeShipping discount is not present in DiscountInfo on the SalesOrder object unless the shipping cost is above 0.

This makes it hard to detect if the current cart is eligible to free shipping, unless you either set the cost of the shipping option (in backoffice) to something above 0, or apply a custom shipping fee in code.

The solution would be to always have the FreeShipping discount info in the DiscountInfo list, if the current cart is eligible, regardless of the current shipping cost.