The intersection seam of the surfaces is coincident.
It doesn't matter much if the surface of the box
behind the object surface is shaded, but in the
case of the bowl the surface of the box behind
the seam is lit.
You can make it look right by putting a bottom
on the bowl...
difference { // bowl
object{testscene__null__ }
plane {y,0.102}
uv_mapping material{_null__ }
}
