The contact owns a 2023 Kia K5. The contact stated while driving at approximately 70 MPH, the transmission unexpectedly downshifted, shuddered, and there was an abnormal sound coming from the vehicle. The Electronic Stability Control Warning Light was illuminated. The vehicle was taken to a dealer where it was diagnosed that the transmission needed to be replaced. The vehicle was repaired; however, the failure persisted. The manufacturer was notified of the failure. The failure mileage was approximately 18,000.