The TSB is for cars manufactured on a specific month only. If your car wasn't manufactured during that month then they basically tell you to go pound sand. The only way you'll get that resolved is to keep raising hell. But as you said, you can return the car when your lease is up. I love my Stinger but their level of service is pretty much crap.
The TSB says it applies to Stingers manufactured prior to May 2018. Mine was January 2018, which was stated on First Kia's service receipt. I had the car brought in for this issue as well as the wiring recall. Screenshot from the TSB attached which clearly states "prior to the dates above"