This may be a known problem with KIA, the fact is you have asked them to fix it and they haven,t. Suggest you advice them you are going to have the vehicle inspected by a vehicle engineer and then you will issue legal action. This may seem an extreme measure but it may be your only option.
Send me a PM with your contact details if you need further information. The fact that they alleegedly checked the vehicle and 3 weeks later the engine let go actually works in your.