@janny13
I have a similar 2020 2LT 3.6L FWD Blazer....and you're correct, it could be the Auto Stop-Start.
Since you didn't mention in your original post if you were on a flat vs inclined road surface, I thought I'd add my experience to the conversation.
The first few times, after purchasing my Blazer, I was caught off guard with the "Hill-Start Assist" feature. I read about it, but the first time I experienced it, the vehicle felt like it was jerking forward.
I hope it's not your transmission...but don't fall for the "sealed for life" / "don't need to change fluid" excuse that might be given to you by the repair facility.
Best wishes...and please let us know what the final diagnosis is (was).