Iron Bars and 3d Export

When Minecraft exports models of iron bars, for some reason some of the texture data is lost.

After multiple test, this is a recurring issue in the 3d export feature. This always happens.

Reproduce: 

Create some sort of structure using iron bars.

Use a structure block to export it as a 3d model

Import the model into a modeling/development program compatible with gltf2

You'll see the error