RAYMARCHING MATERIALS dont work with smooth union operators?

Hi there,
Any raymarchers here? :slight_smile:
i try to put 2 materials on 2 different objects, then union them. no prob with the the min function!
but as soon as i want to smooth union them, the cube gets black including the smooth union intersection with the plane.
i guesss the smooth union operator alters the size of my cube a little so that the material cant be applied correctly, am i right? does anyone have a solution for this?
here is the shader on shadertoy.