I know its been a while since this thread was updated but I recently purchased a '21 Silverado RST and I am experiencing a similar issue... I had a theory that the auto lock was overridden if a door was left open too long so I ran the following test... During the entire test I did not leave the range of the key fob so that may be another variable that was not taken into account.
Unlocked the truck with the driver door handle button. I then opened and left the rear driver side door open for about 40 seconds and then shut it. The auto lock did NOT engage.
I repeated this same experiment for 30 seconds, 40 seconds, 50 seconds, 1 minute... Auto lock DID engage all these times.
I then left the the driver door AND the rear driver side door open for, 30 seconds, 45 seconds, 1 minute... Auto lock engaged all these times.
I wasn't able to test much more since this was done at my office in the morning when i should be working... I'll try more things in the future and comment if I find anything definitive...
