If the head-chop component is set up to hide the head, when you get your avatar 'stuck' in a collider (that thing in VR where your avatar freezes in place when you for example stick your head through a wall or get too close to a table etc) your head and all its children become hidden in cameras, mirrors and shadows in-world.
Reset avatar or reloading it fixes it.
I've only been able to test this in PC VR on an Index.