Stereoscopic postscreen effect UV differences (multi pass)

I tried to strech the image in a postscreen effect by simply multiplying the u texture coordinate with a factor.

However the image comes out differently on each eye. With a factor of 1.5 the Image is compressed to 2/3 on the left eye but to 1/2 on the right.

As I thought and tested the UV space in a postscreen effect should be 0 to 1. Thus I cannot explain the difference in both eyes.

Although I am rendering in multi pass mode I tried to use UnityStereoScreenSpaceUVAdjust which did not change anything.

Here is the shader + script in text form:

 Shader "Hidden/UVOffset"
 {
     Properties
     {
         _MainTex("Texture", 2D) = "white" {}
     }
 
     SubShader
     {
         // No culling or depth
         Cull Off ZWrite Off ZTest Always
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             sampler2D _MainTex;
             half4 _MainTex_ST;
 
             fixed4 frag(v2f_img i) : SV_Target
             {
                 i.uv.x *= 1.5;
                 fixed4 col = tex2D(_MainTex, UnityStereoScreenSpaceUVAdjust(i.uv, _MainTex_ST));
                 if (i.uv.x >1) col = fixed4(0, 0, 0, 1);
 
                 return col;
             }
 
             ENDCG
         }
     }
 }

 [ExecuteInEditMode]
 public class UVOffset : MonoBehaviour
 {
     public Shader Shader;
 
     private Material material;
 
     void OnRenderImage(RenderTexture source, RenderTexture destination)
     {
         if (this.material == null) this.material = new Material(this.Shader);
         Graphics.Blit(source, destination, material);
     }
 }

This is the same code attached as a zip: UVOffsetPostFX.zip

The render Settings are as follows:

I hope anybody can help me with that because it is pretty specific and it seems like few people are using quadbuffered 3d.