My avatar has perfect fullbody without any issues in SDK2, but in SDK3 my viewpoint and head will shift backward to align with my hips for whatever reason, causing the spine to bend backward awkwardly and it ruins the tracking. Maybe this is related to hip flipping but I had to do that because previously there was a slight tilt to the side in my hips and that was the only way to fix it.

EDIT:

The same exact problem occurs regardless of whether or not the hip is flipped, and I already undid the fix and tried following the tracking guide.

There is also another problem with fullbody: avatars will actually move their head and torso to be able to reach when the avatars arms are shorter than the user's real arms. Reaching forward for example will make the avatar lunge forward and bend over, as opposed to the previous behavior where the arms extended as far as possible. The new viewpoint system is likely the culprit for everything and it also causes all default animations to look really awful.

Here are 2 examples, one without calibration and one with: