I have this same
I'm having this same exact issue with my 2023 AT4 (3.0L). I have around 500miles on the truck and my DEF indicator in the truck is only down one line from full. However, my MyGMC app has been showing DEF Range Low since a week after I got the vehicle. Everything else in the app seems to be working fine...sometimes my PSI might be off by 1 or 2, but no biggie. GM tells me the DEF Range Low issue is an OnStar issue since only my app is giving me false info. OnStar has elevated my complaint like 5 times now and gone through diagnostics over the phone with me like 10x. I don't know if its an app issue or if there is something within the truck that isn't feeding correct info to the app, specifically the DEF Range.
I want to clarify that I don't get any warnings or popups in my vehicle cluster or dashboard...the ONLY issue is the app reporting DEF Range Low.