While playing splitscreen, if player 1 is riding a horse or boating, when he bumps into walls or comes into a constricted space, blackness will fill player 2's screen until player 1 is free from constriction or away from the wall. It can cover anywhere from half to all of player 2's screen. I don't think it happens when reversed (player 2 boating or horseback), but I haven't tried enough to say. Basically it makes it frustrating for second player when player 1 is riding or boating as his/her screen is obscured constantly.

This is pretty consistent, almost constant. To replicate just play splitscreen and have player one boat or ride a horse into a tight space. You can see in the screenshot I've uploaded, as I bump a wall on horseback as player 1 (on top), one third to half of player 2's screen (bottom) is blocked by blackness.