After the latest avatar SDK update, the root of the avatar jitters in 3-point tracking. This is affecting all the objects outside the humanoid armature.
The jitter goes away if the avatar goes to the crouch mode, or if "Use Auto-Footsteps for 3 and 4 point tracking" is disabled in the VRC Avatar Descriptor.
I tried to capture the behaviour using a fresh unity project (created with VCC) and the default pilot avatar, with a cube in the root as a place holder.