The creators of it probably didn’t expect that anyone would wear headless head with it, so they didn’t model the inside, though nobody really uses the head with headless head unless they’re making a Halloween avatar.
Thank you for reaching out. After reviewing your report, the behavior you described is intentional and aligns with our design choices. If you have further questions or need clarification, feel free to ask.
Since this post is still getting attention, I’d like to show the reason I believe why the modeler made the inside hollow.
When you view the transparent skull from an angle which covers up a concave part of the mesh, the two layers leave a noticeable overlap. It makes sense for something physical like glass, but it doesn’t look right with something otherworldly like a ghost. Hiding the inside of the skull removes a lot of possible overlaps.