ya sometimes will get those bucket edge artifacts if displacement bounds are 0. this is covered in the manual. try 'em at 0.1, e.g. as with shading rate, this may be over-ridden by the object properties. in the above image, "cast shadows" is unchecked, but I just said that because it's wrong to render without shadows IMVHO. I doubt it will affect the artifacts, unless ya switch to depth-mapped shadows.