Why wont the normals of my addpass terrain shader blend with the first pass?

So I've been deving a custom shader for Unity for quite some time now and have recently discovered a major bug with the blending between the Add pass and the First pass but it only applies in foreward rendering. I think the attached images will make the problem clear.

 Deffered rendering (blending is fine)

Forward rendering (blending is buggy)

I am already certain that the problem is in the normals and not the albedo as I have experimented with disabling the normals of the shader and the problem disappears.

I will provide a truncated version of the shader and a demo scene which still exibits the same bug in forward rendering.

 Shader "Custom/Terrain Shader Test First" {
     Properties {
         [HideInInspector] _Control ("Control (RGB)", 2D) = "red" {}
         [HideInInspector] _Splat3 ("Layer 3 (A)", 2D) = "white" {}
         [HideInInspector] _Splat2 ("Layer 2 (B)", 2D) = "white" {}
         [HideInInspector] _Splat1 ("Layer 1 (G)", 2D) = "white" {}
         [HideInInspector] _Splat0 ("Layer 0 (R)", 2D) = "white" {}
         [HideInInspector] _Normal3 ("Normal 3 (A)", 2D) = "bump" {}
         [HideInInspector] _Normal2 ("Normal 2 (B)", 2D) = "bump" {}
         [HideInInspector] _Normal1 ("Normal 1 (G)", 2D) = "bump" {}
         [HideInInspector] _Normal0 ("Normal 0 (R)", 2D) = "bump" {}
         [HideInInspector] [Gamma] _Metallic0 ("Metallic 0", Range(0.0, 1.0)) = 0.0    
         [HideInInspector] [Gamma] _Metallic1 ("Metallic 1", Range(0.0, 1.0)) = 0.0    
         [HideInInspector] [Gamma] _Metallic2 ("Metallic 2", Range(0.0, 1.0)) = 0.0    
         [HideInInspector] [Gamma] _Metallic3 ("Metallic 3", Range(0.0, 1.0)) = 0.0
         [HideInInspector] _Smoothness0 ("Smoothness 0", Range(0.0, 1.0)) = 1.0    
         [HideInInspector] _Smoothness1 ("Smoothness 1", Range(0.0, 1.0)) = 1.0    
         [HideInInspector] _Smoothness2 ("Smoothness 2", Range(0.0, 1.0)) = 1.0    
         [HideInInspector] _Smoothness3 ("Smoothness 3", Range(0.0, 1.0)) = 1.0
     }
     SubShader {
         Tags {
             "SplatCount" = "4"
             "Queue" = "Geometry-100"
             "RenderType" = "Opaque"
         }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Standard vertex:SplatmapVert fullforwardshadows //finalgbuffer:SplatmapFinalGBuffer
         // Use shader model 3.0 target, to get nicer looking lighting
         #pragma target 3.0
 
         uniform sampler2D _Control;
 
         sampler2D _Splat0,_Splat1,_Splat2,_Splat3;
         sampler2D _Normal0,_Normal1,_Normal2,_Normal3;
 
         half _Metallic0;
         half _Metallic1;
         half _Metallic2;
         half _Metallic3;
 
         half _Smoothness0;
         half _Smoothness1;
         half _Smoothness2;
         half _Smoothness3;
 
         struct Input {
             float3 worldPos;
             float2 uv_Control;
         };
 
         void SplatmapVert(inout appdata_full v, out Input data)
         {
             UNITY_INITIALIZE_OUTPUT(Input, data);
             float4 pos = mul (UNITY_MATRIX_MVP, v.vertex);
             //UNITY_TRANSFER_FOG(data, pos);
 
             v.tangent.xyz = cross(v.normal, float3(0,0,1));
             v.tangent.w = -1;
 
             //data.projPos = ComputeScreenPos(pos);
 
             //data.localPos = v.vertex.xyz;
         }
 
         void surf (Input IN, inout SurfaceOutputStandard o) {
             fixed4 splatControl = tex2D(_Control, IN.uv_Control);
 
             fixed4 splat0;
             fixed4 splat1;
             fixed4 splat2;
             fixed4 splat3;
 
             fixed4 normal0;
             fixed4 normal1;
             fixed4 normal2;
             fixed4 normal3;
 
             float2 _ScaledUV = IN.worldPos.xz / 50;
 
             normal0 = tex2D(_Normal0, _ScaledUV);
             normal1 = tex2D(_Normal1, _ScaledUV);
             normal2 = tex2D(_Normal2, _ScaledUV);
             normal3 = tex2D(_Normal3, _ScaledUV);
 
             splat0 = tex2D(_Splat0, _ScaledUV);
                           splat1 = tex2D(_Splat1, _ScaledUV);
             splat2 = tex2D(_Splat2, _ScaledUV);
              splat3 = tex2D(_Splat3, _ScaledUV);
 
             fixed4 blendSplat = splatControl;
             int b1 = 1;
             int b2 = 0;
             int b3 = 0;
             int b4 = 0;
 
             fixed4 finalCol;
             finalCol  = blendSplat.r * splat0;
             finalCol += blendSplat.g * splat1;
             finalCol += blendSplat.b * splat2;
             finalCol += blendSplat.a * splat3;
             finalCol /= b1 + b2 + b3 + b4;
 
             fixed4 nrm;
             nrm  = blendSplat.r * normal0;
             nrm += blendSplat.g * normal1;
             nrm += blendSplat.b * normal2;
             nrm += blendSplat.a * normal3;
             nrm /= b1 + b2 + b3 + b4;
 
             half finalGloss = dot(blendSplat, half4(_Smoothness0, _Smoothness1, _Smoothness2, _Smoothness3)) / (b1 + b2 + b3 + b4);
 
             fixed Alpha = dot(splatControl, 1);
             nrm = (nrm*2)-1;
 
             o.Normal = float3(nrm.rg, 1);
             o.Albedo = finalCol;
             o.Alpha = Alpha;
             o.Smoothness = finalGloss;
             o.Metallic = dot(blendSplat, half4(_Metallic0, _Metallic1, _Metallic2, _Metallic3)) / (b1 + b2 + b3 + b4);
         }
         ENDCG
     }
     Dependency "AddPassShader" = "Custom/Terrain Shader Test Add"
     Dependency "BaseMapShader" = "Hidden/TerrainEngine/Splatmap/Standard-Base"
 
     Fallback "Nature/Terrain/Diffuse"
 }

I have also attached a scene which clearly illustrates the problem at the following link: https://drive.google.com/file/d/0B7XHf-0bgF2-NjFfRmpLUzR6UzQ/view?usp=sharing

Note: if you require anymore information please feel free to ask.

Thank you in advance.