Okay first off the transmission was rebuilt due to hard shifting and flaring

The rebuild is finished and the truck drives but when shifting into reverse it “slams” into gear. Tech says it will not accept the valve body program and he has been struggling with it for a week

Has an edge tuner on it and I reverted it back to stock tunes but will not hear back from them until Monday. Could the truck possibly need a new cpu or has anyone ever ran into this problem before?