Transmission Gear Slipping problems of the 2018 GMC Acadia

When the vehicle is warm, the transmission appears to slip when shifting from 2nd to 3rd gear. The engine will rev between 500 to 1000 rpm while shifting. During this time (half to full second) the vehicle is not accelerating and does not respond to throttle inputs. The vehicle has been taken into dealerships 4 times for this complaint. At 35000 miles (3rd dealer visit), gm replaced the valve assembly. The problem reoccurred and gm is only offering to replace the same part, or extend the warranty on the transmission, but not both. No one at the dealership or gm technical assistance center has any idea what is actually wrong and is not taking any steps to find the cause of the issue. The service manager has admitted that there is a problem, but gm denied him any help in resolving the issue. They have said that this is not a safety issue, but I do not feel comfortable driving a car that may become unresponsive to accelerator pedal input for a full second. Note that this issue is still covered by the powertrain warranty at this time.