No shadows when looking through a cutout shader

I'm trying to have a custom cutout shader cast shadows (blue arrow), and allow shadows to be seen through the cutout (red arrow). As you can see, none of that is happening right now.

(Its a cube with the cutout shader intersecting a default cube below, viewed from the top with an orthographic camera)

I need a custom shader because the border cutout detail is stored in a separate detail texture. My shader code is below:

 Shader "Custom/TerrainShader" {
     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _Cutoff ("Alpha cutoff", Range(0,1)) = 0.5
         _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
         _DetailTex ("Detail (RGB)", 2D) = "white" {}
     }
     SubShader {
         Tags { "Queue"="AlphaTest" "RenderType"="TransparentCutout" }
         LOD 200
         
         CGPROGRAM        
         #pragma surface surf Standard fullforwardshadows alphatest:_Cutoff
                 
         #pragma target 3.0
 
         sampler2D _MainTex;
         sampler2D _DetailTex;        
         fixed4 _Color;
 
         struct Input {
             float2 uv_MainTex;
             float2 uv2_DetailTex;
         };
 
         void surf (Input IN, inout SurfaceOutputStandard o) {            
             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color * tex2D(_DetailTex, IN.uv2_DetailTex);
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     }
     FallBack "Transparent/Cutout/Diffuse"
 }

It is based on the old Transparent/Cutout/Diffuse shaders, but also samples the detail texture I described.

If I change the Queue to Overlay, it draws on top of the shadow casting and I can see shadows through the cutout (red arrow), but obviously it doesn't cast its own shadows then. I think I have to alter the shadow pass to make it discard the alpha pixels in the detail texture, but that wouldn't solve the problem pointed at by the red arrow, I think.

Any ideas?