Another thing that might explain the disproportionate change at 30 yards could be if you're overdrawing at 30 yards, maybe because it's further and you want to make sure you're coming all the way to full draw, or something. When you overdraw, your drawing elbow moves from behind the arrow to inside the arrow, throwing the shot to the right.
If it's any consolation to you, most people have the opposite problem and would love to exchange it for yours.