I am beginning to suspect that navigating planet-side is not like traversing the surface of a sphere. If so, what shape should I imagine to aid in navigation?

I assumed a sphere, but my instincts keep getting confounded. Various things have made me suspicious: travel time to markers not being what I expect, markers seeming to shift position while flying at 1500u in the stratosphere, and so on.

I have not ruled out my earthbound mind becoming befuddled since I've never circumnavigated a planet for real, especially from a spacecraft. So I performed the following test.

I made a beacon. From the stratosphere, I flew until the beacon flipped from behind me to in front of me. That must mean it was on the farthest side of the world. At that point, it was 2 hours away at 150u. I flew straight down and landed. Without moving, the beacon was reported as 6 minutes away (via Nomad). This makes no sense!

The beacon marker had a tall vertical line, which indicates it was far around the curve of the planet. I had previously found a crash site, and its marker showed it to be 2 hours away, but it had same length line as the beacon.

The beacon marker and crash site marker were in the same 180° slice of the sky. I approached the crash site on my Nomad. The beacon marker and crash site marker converged. My gut tells me that, in this case, they should get farther apart as I approach one. Also, the travel time to the beacon went up from 6 minutes as I approached.