Oofta
Legend
Good point. Wouldn't make any difference in my game because there's no reason to think that Grimmwald couldn't see the orc, or know that he was leaving.Keep in mind that the Opportunity Attack is provoked as the orc is leaving the square. At that point, it is not really "around the corner" anymore, and should be clearly in Grimmwald's view (see the green circle in attached image).
The better "cheesy" question here would be if the orc's direction of travel was reversed. If Kalista cast some fear effect on the orc in front of her and it followed the blue path ending in its shown position, would Grimmwald get an OA as it left the square directly around the corner from him (see the red circle in attached image)? RAW is yes. Personally I would give the orc cover from Grimmwald that far around the corner.
View attachment 136242
Whether it make sense or not, combatants are assumed to have 360 degree awareness except under unusual circumstances. While I sometimes think that's a bit silly, it's also simple and the best way to run the game for me.