Shader problem with multiple prefab instances

I made a prefab with simple cube and applied "Display Normals" shader to check the normal of faces of the model.

If I put one prefab instance into the scene, rotation of the model does not change face colors. But if I put one more instance into the scene the face colors of both two instances change immediately. Face colors varies when I rotate them.

It seems like the face color is decided not by local coordinate but world coordinate with multiple prefab instances. Is this a bug or just a improper setting problem?

 Shader "Display Normals" {
     SubShader{ 
         Pass{
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
 
             // vertex input: position, normal
             struct appdata 
             {
                 float4 vertex : POSITION;
                 float3 normal : NORMAL; 
             };
 
             struct v2f 
             {
                 float4 pos : SV_POSITION;
                 fixed4 color : COLOR;
             };
 
             v2f vert(appdata v)
             {
                 v2f o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.color.xyz = v.normal * 0.5 + 0.5;
                 o.color.w = 1.0;
                 return o;
             }
 
             fixed4 frag(v2f i) : SV_Target{ return i.color; }
                 ENDCG
         }
     }
 }