Starting with some MY2016 GM software, TRIFECTA engineers noted that how the ECM reports manifold pressure through the “SAE defined PID (Parameter ID)” was changed. In prior years, this PID would report an 8 bit value, and that 8 bit value would directly correlate to manifold pressure. This means the ECM could report between 0 and 255 kPa of manifold absolute pressure (MAP).
In more recent times, it's fairly typical to see in excess of 255 kPa of manifold pressure (approx. 22psi at sea level) on turbocharged vehicles, so there were changes made to the way this PID reports pressure (e.g. it reports less than it used to, in exchange for being able to report a higher number, offset and scaling changes).
All of the aftermarket gauges we've seen that calculate “boost” pressure do so by querying the SAE defined PID for barometric pressure, then subtracting it from the MAP. Positive values would be considered “boost” situations. Because of the changes in this MAP PID reporting, these gauges are now making erroneous calculations.
In fact, to work around this problem, internally, we switched away from using the SAE defined MAP PID in our datalogging software to a mechanism that has been consistent for years and allows for greater resolution in its reporting.
But, yes, the issue here is simply the gauge isn't reporting the boost levels properly – on the stock calibration, or the TRIFECTA calibration because it doesn't understand how the SAE defined MAP PID has been changed for the software in this vehicle.
-TRIFECTA SGE Performance Team