How cold was it?
DEF fluid freezes around 12f and depending on how I park (truck sloped nose first) and it freezes, next startup I will get a false reading until it thaws out, this is when ithe tank is close to the 30% left, but still reading OK.
I know on the 2011-2016 trucks, the system holds more than 5 gallons, but due to the stuff freezing, GM says only 5 gallons due to expansion from freezing.
I haven’t looked at the Duramax supplement guide on the 2017+ trucks with the 7 gallon tank, but I am going to assume it is the same.