You made a deposit so you deserve an answer however I don't think you are going to like it. Sportage isn't the only model with this problem, also have it with the Sorento and Sedona. As of now we have no information from Kia as to the cause of this problem.
Things to try if you haven't already would be, (with the lights off) clean the contacts in the bulb socket and put a little more tension on the ground tab in the socket. We have also had better luck using Kia packaged bulbs.
If this answer satisfy you please accept or if you wish I will opt out for someone else to try. I don't think you will get a better solution though.