My spies are "forgetting" where a minor's city is located

[Xebec]

I am playing a game (beta branch on Steam) where I have a minor nearby whose city keeps disappearing from my map.

The minor is pretty good at finding/killing my spies, but if I send 2-3-4 spies on a turn, I'll see their city for the next turn, but then after a turn of being at "0" spies, their city no longer shows on the map.

Is this an intended mechanic? Do mapmakers in 6000 years just erase entries on drawn maps when spies don't report regularly?

[BlueTemplar]

ShEmp is a very complex game - it can be hard to figure out what is going on even with screenshots - please upload a save ?