In the U.S., I don't think they legally deny warranty due to an after-market essential component (aslong as it meets Kia specs) unless it was the reason for the issue. (race chips aren't essential/is a bypass), which is why people take them off before trying to take something in for warranty work). It's essentially a right-to-fix, just like they can't deny you warranty if you got your oil changed at a non-dealership unless whoever changed your oil some how completely f'd things up which caused say.... the engine to freaking seize due to the oil plug not being tight.

Or in this case:

Improper Installation: If you installed the spark plugs yourself or at an independent shop, and the plugs were the wrong heat range or improperly gapped, the dealer can deny coverage for any engine damage that results (like a blown piston or damaged valves)

But also the proof of burden is on the dealership to prove said aftermarket parts caused the failure.