Greyscale Shader Removes UI Masking

I have a image that is being masked so it has transparent curves. When I add a grey scale Mat to the image, the UI Mask component that creates the curves disappears, as shown in the image.

 // Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
 
 Shader "Custom/GreyScaleFill" {
 Properties
 {
     [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
     _Color ("Tint", Color) = (1,1,1,1)
     [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
     _GrayScale("Grayscale Percent Fill", Range(0.0, 1.0)) = 0.0
      _StencilComp ("Stencil Comparison", Float) = 8
          _Stencil ("Stencil ID", Float) = 0
          _StencilOp ("Stencil Operation", Float) = 0
          _StencilWriteMask ("Stencil Write Mask", Float) = 255
          _StencilReadMask ("Stencil Read Mask", Float) = 255
            _ColorMask ("Color Mask", Float) = 15
 }
 
 SubShader
 {
     Tags
     { 
         "Queue"="Transparent" 
         "IgnoreProjector"="True" 
         "RenderType"="Transparent" 
         "PreviewType"="Plane"
         "CanUseSpriteAtlas"="True"
     }
 
     Cull Off
     Lighting Off
     ZWrite Off
     Blend One OneMinusSrcAlpha
 
     Pass
     {
     CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         #pragma multi_compile _ PIXELSNAP_ON
         #pragma multi_compile _ GRAYSCALE_ON GRAYSCALE_OFF
         #include "UnityCG.cginc"
         
         struct appdata_t
         {
             float4 vertex   : POSITION;
             float4 color    : COLOR;
             float2 texcoord : TEXCOORD0;
             
         };
 
         struct v2f
         {
             float4 vertex   : SV_POSITION;
             fixed4 color    : COLOR;
             half2 texcoord  : TEXCOORD0;
         };
         
         fixed4 _Color;
         float _GrayScale;
 
         v2f vert(appdata_t IN)
         {
             v2f OUT;
             OUT.vertex = UnityObjectToClipPos(IN.vertex);
             OUT.texcoord = IN.texcoord;
             OUT.color = IN.color * _Color;
             #ifdef PIXELSNAP_ON
                 OUT.vertex = UnityPixelSnap (OUT.vertex);
             #endif
 
             return OUT;
         }
 
         sampler2D _MainTex;
 
         fixed4 frag(v2f IN) : SV_Target
         {
             fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
             c.rgb *= c.a;
             if (IN.texcoord.y > _GrayScale) {
                 fixed avg = (c.r + c.g + c.b)/3;
                 c.rgb = avg;
             }
             return c;
         }
     ENDCG
     }
 }
 }