The reason is due to raycasts not being able to collide on things that can’t collide with Default. So if you set something to not collide with Default you can run into all sorts of problems. We are exploring ways to make this easier to work with.
For now, the best thing to do is to make another group and put all interactable parts in your game in it (call it like Parts Default or something).Then, have the heads in another group that doesn’t collide with your new Parts Default group. Basically just avoid having something not collide with Default.