Problem with custom shader (ShaderLab + Cg)

Hi. I'm newbie in Cg. It is my first my own shader. It is perfectly works on android and Windows platform. But when we speak about iOs platform, some horrible thing begins. When model with this shader displayed for first time all is ok: but when I disable gameobject and reactivate it again, the surface is seems to be deformed. I'm using OpenGl ES 3 api for iOs, and OpenGl ES 2 api for android. Here is my code:

 Shader "AlphaVideo" {
     Properties{
         _MainTex("Base (RGB)", 2D) = "white" {}
         _Mask("Mask (RGB)", 2D) = "white" {}
     }
 
         SubShader{
         Tags{ "Queue" = "transparent" "RenderType" = "transparent" }
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha
         Lighting Off
 
         Pass
     {
         CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 
 #include "UnityCG.cginc"
 
     struct v2f {
         float4  pos : SV_POSITION;
         float2  uv : TEXCOORD0;
     };
 
     sampler2D _MainTex;
     sampler2D _Mask;
     float4 _Mask_ST;
     float4 _MainTex_ST;
 
     v2f vert(v2f v)
     {
         v2f o;
         o.pos = mul(UNITY_MATRIX_MVP, v.pos);
         o.uv = v.uv;
         return o;
     }
 
     half4 frag(v2f i) : COLOR
     {
         float2 uv2 = TRANSFORM_TEX(i.uv, _Mask);
         float2 uv1 = TRANSFORM_TEX(i.uv, _MainTex);
         half4 base = tex2D(_MainTex, uv1);
         half4 a = tex2D(_Mask, uv2);
         base.a = a.b;
         return base;
     }
         ENDCG
         }
 
     }
 }