You aren’t alone in experience 8-speed tranny issues. My 2017 Sierra has the hard shifting problem in the low end, which is starting to creep up the bands. It also seems to be rough idling more. I had to take mine to another dealer where the mechanics were at least familiar with the problem and didnt waive it off initially as “normal”. After supposedly talking to GMC, they tried a fix which i believe just optimized the shifting points around that seemed to make it go away – except that this “adaptive” transmission never stays set and the problem has returned. No I think the dealer mechs have given up and saying the “problem is now normal”. No, you aren’t crazy and if you have the endurance for it, contact GMC directly and start the process, as well as either keep taking it to the dealer OR switch dealers. We all paid alot for these “professional grade” vehicles. Its F’d up.