Any reason why surface patterns for subcategories are grayed out in the Visibility/Graphic Overrides?

We made a late change and would like to add cladding to the interior side of our window frames, instead of adding this cladding to each window family it would be great if we could just simply change the surface pattern in our view templates. However it seems we can only override the surface pattern of the entire window, including the glass.
any help would be greatly appreciated.
We made a late change and would like to add cladding to the interior side of our window frames, instead of adding this cladding to each window family it would be great if we could just simply change the surface pattern in our view templates. However it seems we can only override the surface pattern of the entire window, including the glass.
any help would be greatly appreciated.
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