When using full body tracking, the spine bone is always aligned with the chest bone and the spine strays straight. This causes the viewpoint to be misaligned when the user's back is not straight, such as when sitting. People have attempted to remedy this by making a "neck fix" to their avatars. This should be able to be fixed by adjusting the weights of the spine bones in the IK setup.

Video of the problem: https://youtu.be/IqUVFuSD5a4?t=64