I have a 2019 GMC AT4. Last year, at 63000 miles, my right side lifters failed. Totally covered by my extended warranty (Zurich) the dealership ended up replacing both right and left side lifters, pistons, cam shaft, etc.
Previous to that, at 55000 miles, I was having the slipping from 1-2 and vibration at 53 mph and 1500 rpms. Dealer flushed/filled the transmission. Problem went away.
Transmission problem reappeared at 70000 miles only the vibration was at 45 mph instead of 53. I had the transmission and torque converter flushed, filter replaced, and filled with AMSOIL SIGNATURE SERIES ATF HP. This was 2 weeks ago, problem is solved.
The tech bulletin has the fix being Mobil 1 ATF HP.