I know it's not completely the same, but I had a 2017 Sierra with almost identical issues. It was only a year old at the time, and about 25000 miles. The infamous 8l90e. Luckily, I had the extended warranty. Took the dealer 2 weeks to figure out it was a 12 dollar part (mode selector sensor/switch I think) that had failed. Had to scan it, and get the engineers at gm to look at the data. Trans didn't understand what gear I was selecting, and when it found itself moving, had an absolute meltdown. Traction control would apply the brakes, it would shift down and redline randomly, slip like crazy in every gear until it reaches cruising speed, and threw every warning light and message it had. Stranded me on the highway 2 hours from town. I traded it in shortly thereafter, because I was pretty sure it took years of life out of that transmission.
So, yes, my 2 cents is it's something electronic, and you probably want a tech 2 to continue your investigation.