I have no idea how that furnace is sensing the static pressure, but when designing/selecting a fan for high altitudes, a correction factor is applied to the static pressure. So, maybe there’s some thing there.
Here’s a brief explanation of how you correct static pressure for temperature and altitude:
https://www.tcf.com/wp-content/uploads/2018/06/Temperature-Altitude-Effects-on-Fans-FE-1600.pdf