It's a predictable result for the same reason rifling marks on a bullet don't change its ballistics appreciably: the air boundary layer over the bullet surface in flight is thicker than the engraving marks. It is certainly thicker than some surface oxidation.
Nose damage will affect long-range accuracy by changing the ballistic coefficient of the bullet and by introducing some wobble in flight. It opens groups by the same mechanism as tilted bullet so, which is by lateral drift due to the center of gravity being off-center in the rifling at the moment it clears the muzzle. But any bias the nose shape creates for off-axis drag is reversed on opposite sides of the rotation, so any amount by which that drag moves the bullet one way it undoes a half a rotation later. This limits its effect. The base, though, if it redirects muzzle blast by its unevenness, is deflected into a larger lateral drift that stays with the bullet all the way to the target.
