UI Billboard Shader - Wrong rotation pivot - Unity

I'm trying to display several Images as billboards. All the Images are under the same Canvas. I'm using a billboard shader I've been working on:

Shader "UI/Unlit/Billboard Transparent Colored"
{
    Properties
    {
        [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)

        _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

        [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
    }

    SubShader
    {
        Tags
        { 
            "Queue"="Transparent" 
            "IgnoreProjector"="True" 
            "RenderType"="Transparent" 
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        Stencil
        {
            Ref [_Stencil]
            Comp [_StencilComp]
            Pass [_StencilOp] 
            ReadMask [_StencilReadMask]
            WriteMask [_StencilWriteMask]
        }

        Cull Off
        Lighting Off
        ZWrite Off
        ZTest [unity_GUIZTestMode]
        Blend SrcAlpha OneMinusSrcAlpha
        ColorMask [_ColorMask]

        Pass
        {
            Name "Default"
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0

            #include "UnityCG.cginc"
            #include "UnityUI.cginc"

            #pragma multi_compile __ UNITY_UI_ALPHACLIP

            struct appdata_t
            {
                float4 vertex   : POSITION;
                float4 color    : COLOR;
                float2 texcoord : TEXCOORD0;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct v2f
            {
                float4 vertex   : SV_POSITION;
                fixed4 color    : COLOR;
                float2 texcoord  : TEXCOORD0;
                float4 worldPosition : TEXCOORD1;
                UNITY_VERTEX_OUTPUT_STEREO
            };

            fixed4 _Color;
            fixed4 _TextureSampleAdd;
            float4 _ClipRect;

            v2f vert(appdata_t IN)
            {
                v2f OUT;
                UNITY_SETUP_INSTANCE_ID(IN);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);

                // Method 1
                /*
                float scaleX = length(mul(unity_ObjectToWorld, float4(1.0, 0.0, 0.0, 0.0)));
                float scaleY = length(mul(unity_ObjectToWorld, float4(0.0, 1.0, 0.0, 0.0)));
                OUT.vertex = mul(
                    UNITY_MATRIX_P,
                    float4(UnityObjectToViewPos(float3(0, 0, 0)), 1) + float4(IN.vertex.x * scaleX, IN.vertex.y * scaleY, 0, 0)
                    //mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0)) + float4(IN.vertex.x * scaleX, IN.vertex.y * scaleY, 0.0f, 0.0f)
                );
                */

                // Method 2
                float4x4 mv = UNITY_MATRIX_MV;

                mv[0][0] = 1.0;
                mv[0][1] = 0.0;
                mv[0][2] = 0.0;

                mv[1][0] = 0.0;
                mv[1][1] = 1.0;
                mv[1][2] = 0.0;

                mv[2][0] = 0.0;
                mv[2][1] = 0.0;
                mv[2][2] = 1.0;

                OUT.vertex = mul(
                    UNITY_MATRIX_P,
                    mul(mv, IN.vertex)
                );

                OUT.worldPosition = OUT.vertex;
                OUT.texcoord = IN.texcoord;
                OUT.color = IN.color * _Color;

                return OUT;
            }

            sampler2D _MainTex;

            fixed4 frag(v2f IN) : SV_Target
            {
                half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;    

                color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);

                #ifdef UNITY_UI_ALPHACLIP
                clip (color.a - 0.001);
                #endif

                return color;
            }
        ENDCG
        }
    }
}

The problem is that when the Image is not at the center of the canvas (0,0,0), the billboard rotation is not around it's pivot, but the canvas pivot. Here is a small gif:

As you can see the Image on the left (2,0,0) is rotating around the same pivot of the left one (0,0,0), the Canvas pivot (0,0,0).

Here are some caps of the Images transforms and the Canvas transform:

I've tried several things but with no success. I'm still learning to programming shaders. Any suggestion is really welcome.