Unity Custom Shader Cannot Update Property In Code

Question

I'm trying to follow a tutorial and set some simple shader properties, but according to the Inspector, they aren't being set. I'm new to shaders, not quite sure what I'm doing wrong.

I have a MeshA that has been assigned MaterialA. The mesh itself is adjusted/generated via code, run through an Editor Script-added button.

The terrainMaterial I am changing in my code below, is a reference to the material ASSET, not this MeshA.

When playing the game, I also spawn (lets say 10) new Mesh Objects, all of which use MaterialA.

The code in game:

protected void UpdateMeshHeights()
{
    Debug.Log($"------------------------- Updating Mesh Heights ------------------------");
    //Shader.SetGlobalFloat("_minHeight", generationData.MinHeight);
    //Shader.SetGlobalFloat("_maxHeight", generationData.MaxHeight);
    terrainMaterial.SetFloat("_minHeight", generationData.MinHeight);
    terrainMaterial.SetFloat("_maxHeight", generationData.MaxHeight);
    Debug.Log($"Shader Min Height: {terrainMaterial.GetFloat("_minHeight")}"); // value correct
    Debug.Log($"Shader Max Height: {terrainMaterial.GetFloat("_maxHeight")}"); // value correct
}

The custom shader code:

Shader "Custom/TerrainShader"
{
    Properties
    {
        _minHeight ("_minHeight", float) = 0.0
        _maxHeight ("_maxHeight", float) = 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

        float _minHeight;
        float _maxHeight;

        struct Input
        {
            float3 worldPos;
        };

        float inverseLerp(float a, float b, float value)
        {
            return saturate((value-a)/(b - a));
        }

        void surf (Input IN, inout SurfaceOutputStandard o)
        {
            float proportion = inverseLerp(_minHeight, _maxHeight, IN.worldPos.y);
            o.Albedo = proportion;
        }
        ENDCG
    }
    FallBack "Diffuse"
}

So when reading the values, e.g. _maxHeight, it shows the value I expect. However, the properties on the material still say 0 (and the material is all white, per the inverse lerp calculation with values of 0 for both a and b).

I used global floats temporarily (without anything in Properties) to check the code is working, I see the shader doing exactly what I expect so the issue is definitely how I'm setting (or not setting rather) the properties.

EDIT: Added extra context