I think the intention is if a slot doesn’t use an item from its local inventory for 16+ cars, it “refunds” and kicks those items out of the inventory. But in this case, it reorders the parts, too.
However, with the aircon slot and others (I’ve noticed seat units in the fit seats slot if you upgrade to napa seats, and regular wingmirrors when upgraded to heated/folding wingmirrors), there’s a mismatch between the items ordered, requirements, and items actually used in the car. And when it “refunds” the unused items by making them disappear, it’s not actually refunding money… plus it’s wasting spare importer room.
A workaround is to research the “Import Priorities” tech and set AirCon’s Import settings for the “Aircon Unit” to “Local only”. Even though it’s a listed “requirement”, the cars still go through the slot.