The problem isn't a spike, it's the whole "target boost" in the stock ecu....when you go up in altitude the target boost decreases. So to run 26psi at altitude you have to target 30psi....then the calculated target will be 26.....but when u go down in altitude that target boost will start increasing. So if you're running 26 at 5000 feet and drive to sea level u will target 30.
If you're tuned at 26psi at sea level and drive up in altitude to 6000 ft your boost target will drop to 22psi.
The problem is tricking the target boost at altitude then driving to sea level.
There are other Compensations in there to correct it, but the tuner would basically be guessing at the wgdc correction unless u actually logged the car at the various altitudes.