custom entities doesn't render

I'm trying to create custom entities for an addon, and as an attempt to test this, I tried to create a test addon with some cow skin variants, but when I create a world with this test addon, I have noticed that ONLY the cow variants with custom skins don't get rendered. I don't know if is an actual bug or is only an error in the addon.

I've attached the test addon and a screenshot (in the screenshot, you can see that only appears the cow shadows where the cows with skin variants also should appear)