Unity Surface Shader Using "worldNormal" While modifying Normals Turns Shader Black

So I was writing a simple triplanar mapping shader and while doing so encountered a bug where when using IN.worldNormal in code while writing to o.Normal would result in the albedo turning black as can be seen below: https://ibb.co/n1DHC8
Here is the complete shader code (bug tested on webGl as well as Windows DX11):

Shader "Custom/NormalMappedTest" {
Properties {
    _Color ("Color", Color) = (1,1,1,1)
    _MainTex ("Albedo (RGB)", 2D) = "white" {}
    _NormalMap("Normal Map", 2D) = "bump" {}
    _Glossiness ("Smoothness", Range(0,1)) = 0.5
    _Metallic ("Metallic", Range(0,1)) = 0.0
}
SubShader {
    Tags { "RenderType"="Opaque" }
    LOD 200

    CGPROGRAM
    // Physically based Standard lighting model, and enable shadows on all light types
    #pragma surface surf Standard fullforwardshadows

    // Use shader model 3.0 target, to get nicer looking lighting
    #pragma target 3.0

    sampler2D _MainTex;
    sampler2D _NormalMap;

    struct Input {
        float2 uv_MainTex;
        float2 uv_NormalMap;
        float3 worldPos;
        float3 worldNormal; INTERNAL_DATA
    };

    half _Glossiness;
    half _Metallic;
    fixed4 _Color;

    // Add instancing support for this shader. You need to check 'Enable Instancing' on materials that use the shader.
    // See https://docs.unity3d.com/Manual/GPUInstancing.html for more information about instancing.
    // #pragma instancing_options assumeuniformscaling
    UNITY_INSTANCING_BUFFER_START(Props)
        // put more per-instance properties here
    UNITY_INSTANCING_BUFFER_END(Props)

    void surf (Input IN, inout SurfaceOutputStandard o) {
        // Albedo comes from a texture tinted by color
        fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;

        //Even just having this here causes the bug         
        float3 wnrm = abs(normalize(IN.worldNormal));

        float3 nrm = UnpackNormal(tex2D(_NormalMap, IN.worldPos.xz));

        o.Albedo = IN.worldNormal;// c.rgb;
        o.Normal = nrm;
        // Metallic and smoothness come from slider variables
        o.Metallic = _Metallic;
        o.Smoothness = _Glossiness;
        o.Alpha = c.a;
    }
    ENDCG
}
FallBack "Diffuse"
}

Any help would be appreciated, thanks in advance, - Thomas