Shader fails when using image effects in forward rendering

Unity Engine
1

I made a simple shader (it basically just renders the backfacing triangles).
But it doesn’t work when I use image effects in forward rendering, with a shadow casting light. (works in deferred)

Can anyone explain me why, seeing as it’s such a simple shader, I don’t see why it doesn’t work.

here’s the (simplified) shader:

Shader "Custom/Test" 
{ 
    Properties 
    {
        _MainTex ("bidon bedrukking", 2D) = "bidon bedrukking" {}
    }

    Subshader 
    { 
        Tags 
        { 
            "RenderType" = "Opaque" 
        }

        LOD 300
        Cull Front

        CGPROGRAM
        #pragma surface surf Lambert
        #pragma target 3.0
        #pragma only_renderers d3d9 

        sampler2D _MainTex;

        struct Input 
        {
            float2 uv_MainTex;
        };

        void surf(Input IN, inout SurfaceOutput o) 
        {
            float2 uv = IN.uv_MainTex;    
            fixed4 tex = tex2D(_MainTex, uv);
            o.Albedo = tex;
            o.Alpha = 1;
        }

        ENDCG
    }

    FallBack "Diffuse"
}
2

I noticed it also works when I disable shadows.

So apparently it doesn’t work when I use image effects in forward rendering, with a shadow casting light.
(You might think that sounds fairly specific, but that’s actually fairly default.)

Edit:
I reported this as a bug.