I’m having a similar situation. I bought a 2020 At4 in August while it was still in transit to the dealership. When I went to pick it up two weeks later it had trouble shifting into reverse when they were getting it out of the wash bay. Service dept called gm who said to have the whole transmission replaced. Fast forward a month and I still don’t have my truck. Got an update the replacement trans has been built but not shipped yet. Having mixed feelings now about taking delivery of it after it’s fixed. Should I get out and start from scratch?