Refraction Surface Shader Upside down

Hello everyone,

I have a surface refraction shader which works great when my camera is set to deferred lighting. Weirdly enough though, as soon as I switch to forward rendering, the shader refracts the things behind it upside down. I don't know much about shaders, but I'm pretty sure it has something to do with the "Render Texture Coordinates" section in this post.

Here is the shader:

 Shader "Effects/Distort" {
     Properties{
         _Refraction("Refraction", Range(0.00, 100.0)) = 1.0
         _DistortTex("Base (RGB)", 2D) = "white" {}
         _ObjectVisibility("Visibility", Range(0.00, 1.00)) = 0.05
     }
 
         SubShader{
 
         Tags{ "RenderType" = "Transparent" "Queue" = "Overlay" }
         LOD 100
 
         GrabPass
     {
 
     }
 
     CGPROGRAM
 #pragma surface surf NoLighting
 #pragma vertex vert
 
         fixed4 LightingNoLighting(SurfaceOutput s, fixed3 lightDir, fixed atten) {
         fixed4 c;
         c.rgb = s.Albedo;
         c.a = s.Alpha;
         return c;
     }
 
     sampler2D _GrabTexture : register(s0);
     sampler2D _DistortTex : register(s2);
     float _Refraction;
 
     float _ObjectVisibility;
 
     float4 _GrabTexture_TexelSize;
 
     struct Input {
         float2 uv_DistortTex;
         float3 color;
         float3 worldRefl;
         float4 screenPos;
         INTERNAL_DATA
     };
     struct appdata_t {
         float4 vertex : POSITION;
     };
 
     
     void vert(inout appdata_full v, out Input o) {
         UNITY_INITIALIZE_OUTPUT(Input, o);
         o.color = v.color;
     }
 
     void surf(Input IN, inout SurfaceOutput o) {
         float3 distort = tex2D(_DistortTex, IN.uv_DistortTex) * float3(IN.color.r,IN.color.g,IN.color.b);
         float2 offset = distort * _Refraction * _GrabTexture_TexelSize.xy;
         IN.screenPos.xy = offset * IN.screenPos.z + IN.screenPos.xy;
         float4 refrColor = tex2Dproj(_GrabTexture, IN.screenPos);
         o.Alpha = refrColor.a;
         o.Emission = refrColor.rgb * (1+_ObjectVisibility);
     }
     ENDCG
     }
 }

The previously linked post talks about inserting the following snippet somewhere:

 #if UNITY_UV_STARTS_AT_TOP
 if (_MainTex_TexelSize.y < 0)
         uv.y = 1-uv.y;
 #endif

I probably have to change _MainTex_TexelSize to _GrabTexture_TexelSize, but that's pretty muc all I know. Where do I need to insert this snippet to get it working?

Thanks for your help in advance!