Shader Only Works on "Fantastic"

Hello, I have a simple shader that will mask a 2d texture, using an alpha texture, drawn OnGUI. It works fine in the editor. However, when deploying to a PC standalone or web player, the result is a completely invisible texture.

It seems that if the quality is changed from "Good" to "Beautiful" or "Fantastic" in the pc standalone deployment, the shader will also work. Why is this the case? How can we get this shader to work on lower qualities?

Here is the shader in question:

 Shader "MaskTexture" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Mask ("Culling Mask", 2D) = "white" {}
         _Cutoff ("Alpha cutoff", Range (0,1)) = 0.1
         _Color ("Main Color", Color) = (1,1,1,1) 
     }
     SubShader {
         Tags {"Queue"="Transparent"}
         Lighting Off
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha
         AlphaTest GEqual [_Cutoff]
         Pass {
             SetTexture [_Mask] { matrix [_Matrix] combine texture }
             SetTexture [_MainTex] { ConstantColor [_Color] matrix[_TexMatrix] combine texture * constant, previous }
         }
     }
 }