I suspect one of the most recent patches has made physbones stop syncing. It appears to be a property of the avatar - some avatars simply do not sync their grabs, whether they are grabbing their own or other players physbones and no matter whose physbones they are grabbing.

This is consistant across worlds/restarts/computers. My initial guess was that it could be based on performance rank, but I'm not entirely sure because some of my avatars with very few physbones also fail to grab, including unmodified booth uploads.