I'm uncertain when this started, but in a hybrid fullbody-tracking setup it appears that 3-point tracking is still active locally. I'll provide video comparisons below, but it seems as if from the spine-up(including head and hands), avatars will still IK as if they were in 3-point tracking, while the rest of the avatar is in 6-point tracking.

This leads to unintended results, such as torso stretching and limbs collapsing where they shouldn't. I believe this is a much older bug that people haven't caught because most people who care about doing hybrid setups do it because they can't do a non-hybrid one. I wonder if it's been excused by users as just bad IK, as many of the people I know who complain about certain IK issues are also hybrid users.

My main Index + controllers with vive trackers, and the hybrid setup is Quest 2 with vive trackers.

In the videos below, you can see the different effects that arm extension causes on the torso and head between the two setup variants. It seems maybe the 3-point and 6-point systems are mixing when in hybrid.

Hybrid FBT Example :

Standard FBT Example :