I have faced a very serious issue yesterday with my DJI Phantom 4 Pro drone, the GPS signal quality drops rapidly when the drone is hovering at high altitude but has stronger GPS signal when it is at lower altitude, which is very strange, as i normally experience the opposite.

The drone was flown in a big open field, so there are no tall buildings surrounding it, the vicinity is completely clear.

The flight data log download link is below, as you can see, the GPS signal appears to be strong when the drone is at lower altitude, but the GPS signal quality rapidly degrades as the drone flies higher, in some instances the GPS signal even drops to zero suddenly, when i reduced the altitude of the drone, the GPS signal quickly strengthens again.

Please also note that i have tried to fly at this same location using 2 other similar Phantom 4 Pro drones and all 3 drones experience the same problem!

Can DJI or someone here shed some light on what weird voo-doo magic is going on here?

Attached flight log.

Something is wrong with the flight data file you posted.
It comes out like this:
https://www.phantomhelp.com/logviewer/DDJ0A6RM8OCQ8WBDUL05/

Try again but don't zip the txt file