The shader appears to miscalculate the mesh normals for certain equipped armor pieces, causing them to invert under certain conditions. This results in the armor appearing partially invisible.

For example, the hat, pants, and boots from the leather armor set all have inverted normals unless the torso is also equipped. In that case, the pants' normals are correctly calculated, but the torso, hat, and boots remain inverted. It's a rather peculiar behavior.

While I'm not very familiar with OpenGL, I hope this information helps in tracking down the issue.

Thanks for your work!