Updating a materials "_Glossiness" does not update the smoothness value

I have a material (Standard Shader (Metalic Alpha)) in Unity (2019.4 set for iOS build) that I am trying to update the smoothness of, via script.

 mat.material.SetFloat("_Glossiness", 0.5f);

Now this works when the material is pretty much standard and with just an albedo texture assigned to it. The issue arises when I apply a Metallic texture to the material (Either during runtime or beforehand). The script runs and updates the "_Glossiness" value, which I know, as I can print the values to the console and also see them in the debug inspector window. What it doesn't do though, is update the smoothness value in the inspector and there is no visual change in either the Scene/Game views.

Can anyone explain what is going on here? I am at a loss...

Extra Information:

If I run the application and update the Metallic texture through the debug window (assigning it directly to the "_MetallicGlossMap") The above script continues to work as expected and the smoothness updates.