[TF2] "WeaponSkin" material proxy does not consistently set basetexture(s) for Decorated/Warpaintable items

[Voids29]

When creating custom models for items that can have war paints applied to them, it is sometimes ideal to disable the ability of said weapons to have war paints visibly applied to them. This can almost be fully accomplished by simply not including "WeaponSkin" in the material proxies of the custom model's .vmt file. Unfortunately, this does not consistently work with all war paints and/or decorated weapons. War paints such as the Dragon Slayer, Yeti Coated, Winterland Wrapped, Coffin Nail, Dressed to Kill, and many more that I could list do NOT use the defined default basetexture(s) (i.e. the texture file defined in the $basetexture paramter) when the "WeaponSkin" proxy is absent.

I have included screenshots demonstrating this issue with the Autumn Mk.II and Yeti Coated war paints:

This is a test model I have rigged. The model is comprised of two squares, each using their own material. The red square does NOT use the "WeaponSkin" proxy, and the blue square uses "WeaponSkin" proxy. There is currently no war paint applied in this screenshot

Below demonstrates the expected behavior of the "WeaponSkin" proxy. This screenshot shows that the test model has the Autumn Mk.II warpaint applied to it. The blue square's texture is replaced by the warpaint, while the red square's is not. This is expected, as the blue square's material is telling the game that it should be used for the basetexture of the warpaint .

Now this is where the behavior of the "WeaponSkin" proxy becomes inconsistent. Here we have the Yeti Coated warpaint applied to the test model. As you can see, the warpaint is replaces the textures of BOTH squares in this screenshot. The reason this is unexpected behavior is because the red square's material does NOT say it is should be used as the basetexture of the warpaint, while the blue square's material does.

Below I have zipped the test set up used in the screenshots. This test set up can easily be used by dragging the root directory of the zip into Team Fortress 2's custom folder. The model that is being replaced is the default scattergun.

warpaint_test.zip