error when using exploitation treaty

That's strange. First time someone reports something like this. What do you mean by "using the exploration treaty" ?

I think that is just the graphics reloading with your new "found" territory 

Ahh, yeah. Once all the territory is revealed does the flickering stops ?

cannot tell you as I will never trade my maps with my victims but way back in the 20th century when I did silly things like trade with my enemies the maps would flicker when I did. 

When I use the exploration treaty, the enemy is supposed to appear, but instead, it just keeps flashing.

And if you end the turns that doesn't fix it ? If so, can you share a save file showcasing this ? You can find them in %userprofile%\Documents\My Games\GalCiv4\Saves and share them over on discord at https://discord.gg/galciv or using your preferred file sharing method.