Local position in vertex shader?

I've written a fragment shader to tint a textured and transparent (in places) plane that I have. The idea being that two colours can be supplied and the plane will go from the first colour at the very bottom to the second colour at the very top in a gradient like fashion. The problem is that when I have multiple instances of the plane (with the same material - for optimisation) and they are at different y heights then the y values used in the shader do not appear to be local to each object which messes up the shading - the below image should make things clearer.

In this image all of the grass instances should be bluey green at the bottom and pink at the top (they share a material - they're just at different heights in the scene) - but alas, only the middle one is!

Can anyone see what I'm doing wrong in the below shader that I've written?

 Shader "Unlit/Vertex Multi Color" {
     Properties {
         _Color ("Bottom Color", Color) = (1,1,1,1)
         _Color2 ("Top Color", Color) = (1,1,1,1)
         _Scale ("Scale", Float) = 1
         _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
     }
      
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         LOD 100
        
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha
        
         Pass {
             CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
                
                 #include "UnityCG.cginc"
                
                 struct appdata_t {
                     float4 vertex : POSITION;
                     float2 texcoord : TEXCOORD0;
                 };
      
                 struct v2f {
                     float4 vertex : SV_POSITION;
                     half2 texcoord : TEXCOORD0;
                     fixed4 col : COLOR;
                 };
      
                 sampler2D _MainTex;
                 float4 _MainTex_ST;
                 fixed4 _Color;
                 fixed4 _Color2;
                 fixed  _Scale;
 
                 v2f vert (appdata_t v)
                 {
                     v2f o;
                     o.col = lerp(_Color,_Color2, v.vertex.y * _Scale);
                     o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                     o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
 
                     return o;
                 }
                
                 fixed4 frag (v2f i) : SV_Target
                 {
                     fixed4 col = tex2D(_MainTex, i.texcoord) * i.col;
                     return col;
                 }
             ENDCG
         }
     }
  
 }
  

Any help would be much appreciated, I'm really tearing my hair out over it!

Thanks,

George