Spot light shows no shadow on invisible plane.

I'm trying to use spot light to cast shadow on invisible plane but can't make it work. I found that directional light can cast shadow on both visible and invisible plane while spot light only shows shadow on visible plane.

directional light + lit plane

directional light + unlit & shaderMultiplier plane

spot light + lit plane

spot light + unlit & shaderMultiplier plane

material of invisible plane

material {
    name : shadow,
    shadingModel : unlit,
    shadowMultiplier : true,
    blending : transparent,
    transparency : twoPassesOneSide,
}
fragment {
    void material(inout MaterialInputs material) {
        prepareMaterial(material);
        material.baseColor = vec4(0.0, 0.0, 0.0, 0.5);
    }
}