Custom cave walls material behaves weirdly

[Cleocyde]

when exporting custom caves, walls also appear on the sides of floor, and is more visible when the terrain is more abrupt (here, the floor is supposed to be only black concrete)

the same happens on ceilings, and it's even more noticeable.
white is walls, black is ceiling, and it does this on the ENTIRE ceiling, not just near the actual walls