My runners unit was out exploring and on this high peak, they couldn't see a stretch of land that is clearly below them and closer, but could see other land that was *further* below them and *further* away.

Yes, I agree. 

Line of sight in combat can be really weird at times too due to terrain. Not logical and intuitive at times.

I took another step closer, and it revealed the land there. I am lower in altitude (even with it now), but I can see all of it clearly.