I have had the exact same problem on my 2020 - 2500HD AT4. My truck had an SCA 3.5" lift kit on it with 35" tires. SCA installed a speedometer calibration device in the truck for bigger tires. The local Chevy dealer found a GM Tech Tip that attributed the behavior to the calibration device. At first, I did not believe them; however, SCA immediately confirmed that they had recalled the original module and they overnight shipped me a replacement module. It immediately fixed the issue. I will post if the issue comes back.
The GM bulletin is #pit5752B Sept 14, 2021.
