While I agree that it might be a graphics card issue, I too am experiencing the same exact thing. 1300+ hours in the game, never have had any graphics issues
UNTIL downloading the Leader Pass. It seems to be a glitch with how forests and rainforests in the fog area where you have scouted are rendering.
@AnnHull , can I take a guess and say you are playing on a Mac? If anyone has any insight on a possible fix that would be helpful.