I had previously used a client known as IK tweaks to readjust my shoulders/hip/spine/etc. while in FBT so that my viewpoint wouldn’t enter the chest, but force the head to follow it while laying down. This happens with ALL avatars, laying down, or sitting down in some scenarios.
Any help/confirmation of future IKTweaks made by VRChat would be incredibly appreciative!
Noah
Did you find a solution for this?
Please don’t necro 2 year old threads! 
Using the “Lock Head” IK option*, you can lock your viewpoint to your head. This means your hip won’t be precise anymore because we must shift around your hip position to ensure your back doesn’t bend or stretch unnaturally.
You can try using “Lock Both”, but it only works well if you have an armature properly set up for it.
* This option is available in the Quick Menu Gear tab when FBT is detected.