Something interesting I came across that validated what was shared by members here was this for the new 2020 Chevy/GMC HDs:
"Sierra Heavy Duty models equipped with the Duramax Diesel also feature an integrated engine after-run cool cooling function, which can continue to idle the engine after extended and exhaustive periods of use, allowing for proper cool-down of vital underhood components."
https://www.gmc.com/gmc-life/trucks/...ailer-like-pro
Nice they are building in intelligence around that important need for the health of the engine. I assume that runs on its own after you "turn off" and exit the truck?
Given it is the same L5P Duramax as the current gen, it would seem to indicate this was in fact a concern with the current gen as well and should be given the appropriate attention.