So, I'm pretty sure this is a big problem here. When in fullbody, and you lay down, the Chest seems to follow the head way too much. I believe this comes with having the "Neckfix" and maybe having the spinebone=0. Even models without the "Neckfix" still have the same problem. Even models that are not FBT ready are also having problems. The normal avatars also have this problem too. I'm not sure what might have caused this, but I predict that they modified the weight painting from the chest to the headset when we lay down, but it looks like it's not working as it's intended.

A prediction was made here where I put my hand in the middle of my chest IRL, and move my head side to side, and it seems that the chest "moves" itself. ( https://gyazo.com/8c8f3c729f85119948aba3744ec75365 )