The contact owns a 2018 Ford F-150. The contact stated while driving at an undisclosed speed, the accelerator pedal was depressed; however, the vehicle jerked while in first gear and the transmission unexpectedly downshifted. No warning light was illuminated. The vehicle was taken to the dealer however, no cause for the failure was found. The vehicle was not repaired. The contact stated that the failure persisted. The manufacturer was made aware of the failure and advised the contact to report the failure to the NHTSA Hotline. The failure mileage was approximately 17,000.