MyGMC app

I get what you mean — it can be pretty frustrating when the app features don’t fully match what you expect after getting a module replacement. From what I’ve seen with the MyGMC app, some vehicles only allow basic remote start through the app, but the actual A/C or climate controls are still handled automatically by the vehicle’s last settings rather than being adjustable directly in the app. So when you remote start, the truck usually runs the climate system based on the previous configuration or automatic climate settings.
You might want to double-check a couple of things:

Make sure your vehicle supports full remote climate control in the MyGMC app (some trims don’t).

Confirm that Remote Climate Settings are enabled in the infotainment system.

Sometimes the app needs to be re-synced after a module replacement, which a dealer or OnStar support can help with.

As for updates, a lot of owners have mentioned that OTA infotainment updates don’t roll out very frequently, and sometimes they’re only installed during dealer service visits rather than pushed directly to the vehicle.
I ran into a similar discussion while searching around and found some helpful troubleshooting info on this website it has a few guides that might help explain why some app features behave differently depending on the vehicle configuration.
Hopefully GM improves the app controls in future updates because having remote start without full climate control options does feel a bit limited.