The tire is presumably rigid so it shouldn’t expand like a weather balloon with increasing altitude and the pressure inside won’t change. However your tire gauge has a diaphragm or transducer which works against ambient pressure. It reads higher at higher altitude. However, the TPMS should read absolute pressure. As I live at 5000 ft, this would explain why the TPMS reads 2-3 lbs lower than the gauge when the tires are “cold” and the ambient temperature is about 20C.And if you live at 7000 ft like me?....the inside the tire sensor can’t compensate for the reduced air pressure surrounding the tire. My tires at a sensor reading of 32psi are firmer than if I was in Houston at the same sensor reading.