When leaning backwards or sideways in an avatar on IK2, the spine and chest bone joint does not bend at all forwards or sideways. This is only a problem with the head + hips locked as well(although my avatar has great proportions, so that be an issue). This makes certain leaning positions look very wrong or broken. To fix, I tried messing around with the armature of my model, including having the hip root positioned perpendicular to the thigh bone's roots as well as trying with the hip above the thighs. I also tried angling the spine backwards, moving the viewpoint backwards/forwards, and even added a upper chest bone(haven't painted it yet, just wanted to see if that changed how the IK calculated bending, which it didn't). I found the least breakage to be with a straight spine and hips + things perpendicular, although small variations didn't change it terribly much. The best case scenario on IK-2 has the chest and spine bones be completely straight when leaning backwards, but the worst has them crunch backwards(instead of forwards like it should). I have tried multiple different avatars, and the issue seems to range from okay to atrocious depending on the rigging/weight painting, but mostly the flexibility of the user, as this issue only seems to affect people who can lean/arch far backwards.

[pictures] (all my avatar)