Possible fix!!!
My 2021 Sierra 1500 AT4 failed to start twice in the last month. Each time I waited several minutes and it eventually started. I just got back from the dealer and they found two codes in the history (P0300 and P12A6). They referenced service bulletin #20-NA-218 and reprogrammed the ECM.
supposedly this is the fix for the starting issues. I will update if it happens again.