Hello!
Apologies for the confusion, but this is not how Allow Material Override works, or how it's intended to work. It's just an option to expose certain material properties that are normally not customizable per material. See https://docs.unity3d.com/Packages/com.unity.shadergraph@17.3/manual/surface-options.html
If you want to specify a "default" material that can be inherited from, ala. a prefab, I suggest you use Material Variant:s https://docs.unity3d.com/6000.1/Documentation/Manual/materialvariant-concept.html