The head IK is locked to a very limited rotation when in half body tracking.
The avatar can only look upwards at about maybe 45-60 degrees at most depending on it's rigging. Fullbody users can spin their head any way they want. Why is this limitation on half-body (or desktop for that matter)? There's no reason to limit the head rotation. If someone is going to do something that might do unnatural things with their neck that's their business...
....
but when i can't even look directly up at someone that is hugging me from behind, that is a little ridiculous.
The issues only get worse with some prone poses, which pull the head to even more limited angles due to the spine and chest rotation.
I was shocked there wasn't already a canny for this. The IK Tweaks mod had this feature and it was used quite a bit. This probably doesn't get much vocal recognition because it's hard to recognize and identify the social disconnects happening under these circumstances.