Weird Glitch

Grayscale.shader

Shader "Hidden/Custom/Grayscale"
{
	HLSLINCLUDE

#include "Packages/com.unity.postprocessing/PostProcessing/Shaders/StdLib.hlsl"

	TEXTURE2D_SAMPLER2D(_MainTex, sampler_MainTex);
	float _Blend;



	struct VaryingsExtra
	{
		float4 vertex : SV_POSITION;
		float2 texcoord : TEXCOORD0;
		float2 texcoordStereo : TEXCOORD1;
		float2 texcoordStereo2 : TEXCOORD2;
	};

	VaryingsExtra Vert(AttributesDefault v)
	{
		VaryingsExtra o;
		o.vertex = float4(v.vertex.xy, 0.0, 1.0);
		o.texcoord = TransformTriangleVertexToUV(v.vertex.xy);

#if UNITY_UV_STARTS_AT_TOP
		o.texcoord = o.texcoord * float2(1.0, -1.0) + float2(0.0, 1.0);
#endif

		o.texcoordStereo = TransformStereoScreenSpaceTex(o.texcoord, 1.0);
		o.texcoordStereo2 = o.texcoordStereo;

		return o;
	}




	float4 Frag(VaryingsExtra i) : SV_Target
	{
		float4 color = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, i.texcoordStereo2);
		float luminance = dot(color.rgb, float3(0.2126729, 0.7151522, 0.0721750));
		color.rgb = lerp(color.rgb, luminance.xxx, _Blend.xxx);
		return color;
	}

		ENDHLSL

		SubShader
	{
		Cull Off ZWrite Off ZTest Always

			Pass
		{
			HLSLPROGRAM

				#pragma vertex Vert
				#pragma fragment Frag

			ENDHLSL
		}
	}
}