May be a problem specific to your pump. have you spoken to the dealer or contacted manufacturer?
to outline the differences,
After opening the nut and tapping the stem to unstick the valve,
Presta valves open with the air pressure from the pump being greater than the
air pressure in the tire-tube. they are the most common type valve for high pressure
bike tires

Schrader valve cores have a spring in them, and the pump head needs a pin to keep the pin in the valve core open.,
more common on low pressure mountain and junior bike wheels,

Perhaps your initial reading is the pressure in the hose and not reading the tire?

though I have a tire gage off the pump, the 2 pumps i have:
from Medai, and Silca, have been pretty reliable.

Just a long ways down to the floor, and my eyesight is not as good ,
My now being over 60.