Why does my Image effect shader output gets darker over time?

For some reason, when I use my main texture inside my shader it, over time, gets darker. Before: After Seconds of Playing:

This is the shader code.

 Shader "Hidden/Vintage"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
         _Perlin ("Perlin", 2D) = "white" {}
         _Degrade ("Degrade", 2D) = "white" {}
         _Under ("Undertexture", Color) = (0,0,0,0)
         _Darker ("Darkertexture", Color) = (0,0,0,0)
 
         _Size ("Size", Range(0,2)) = 1 
         _Vig ("Vig", Range(0,1)) = 1 
         _VigIntensity ("Vignette Intensity", Float) = 15
         _DegradeSize ("Degrade", Float) = 1 
 
         _NoRed ("No Red", Range(0,1)) = 1 
 
     }
     SubShader
     {
         // No culling or depth
         Cull Off ZWrite Off ZTest Always
         Tags {"Queue" = "Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
         Blend Off
         //Blend DstColor SrcColor
         //AlphaToMask On
 
         Pass
         {
             CGPROGRAM
 
             #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
 
             float4  _MainTex_ST;
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 vertex : SV_POSITION;
                 float4 screenPos : TEXCOORD1;
             };
 
             v2f vert (appdata v)
             {
                 v2f o;
                 o.vertex = UnityObjectToClipPos(v.vertex);
                 o.uv = v.uv;
                 o.screenPos = ComputeScreenPos(o.vertex);
                 return o;
             }
             
             sampler2D _MainTex, _Perlin, _Degrade;
 
             float _Size, _DegradeSize, _Vig, _NoRed, _VigIntensity;
             fixed4 _Under, _Darker;
 
             fixed4 frag (v2f i) : SV_Target
             {
                 //float2 newUV = TRANSFORM_TEX(i.uv, _Perlin);
                 fixed4 per = tex2D(_Perlin, i.uv);
                 fixed4 deg = tex2D(_Degrade, i.uv * _DegradeSize + _Time[0]);
                 fixed4 col = tex2D(_MainTex, i.uv);
                 fixed4 tCol = tex2D(_MainTex, i.uv);
 
                 //return per;
 
                 float2 sp = (i.screenPos.xy/i.screenPos.w);
    
                 sp *=  1.0 - sp.yx;   //vec2(1.0)- uv.yx; -> 1.-u.yx; Thanks FabriceNeyret !
     
                 float vig = sp.x*sp.y * _VigIntensity; // multiply with sth for intensity
     
                 vig = pow(vig, _Vig); // change pow for modifying the extend of the  vignette
 
                 vig = vig;
 
 
                 fixed3 BColor = fixed3(1,1,1);
 
                 //if(tCol.r > 0.01 && tCol.g < 0.2 && tCol.b < 0.2 && _NoRed > 0)
                 //    col.rgb =  _Under.rgb * (col.r);
 
 
                 col = lerp(_Under, col, col.a);
                 return lerp(col, fixed4(0,0,0,.5), vig);
 
                 fixed4 OUT = col;
 
                 OUT = lerp(OUT, _Darker, (vig * (1-per.b) * _Size) / _DegradeSize);
 
                 //fixed4 BColor = fixed4(0,0,0,1);
 
                 //OUT.a = lerp(OUT.a, BColor, vig/2);
 
                 //OUT.rgb = OUT.rgb * (OUT.a) + _Darker.rgb * (1-OUT.a);
                 
                 //OUT.rgb = OUT.a;
 
                 //OUT.a = 1;
 
                 if(tCol.r > 0.01 && tCol.g < 0.2 && tCol.b < 0.2 && _NoRed > 0)
                     tCol.rgb = 0;
 
                 OUT.rgb += tCol.rgb * 0.4;
 
                 return OUT;
             }
             ENDCG
         }
     }
 }

I am using Graphics.Blit to get the camera's render texture, I have a feeling that it is a blending issue, however I have no idea what is needed to fix this. I would like to have it look like the first picture and for it not to get darker when I play the game. Thanks.