Suppose we have some GameObjects on an avatar. Among them are two GameObjects, henceforth called A and B.

GameObject A is controlled by some VRCConstraint. None of A's children or parents are Physbones.

GameObject B is controlled by some VRCConstraint with GameObject A as one of its sources. Additionally, some of GameObject B's children are Physbones.

Expected behaviour: The VRCConstraints on GameObject A and B both execute before Physbones do.

Observed behaviour (in SDK 3.7.3 and 3.7.4): The VRCConstraints on GameObject A and B both execute after Physbones do, resulting in unexpected forces acting on the Physbone simulation.

Additional note: Removing the VRCConstraint from GameObject A, or removing GameObject A from the sources of the VRCConstraint on GameObject B, causes the VRCConstraint on GameObject B to execute before Physbones as expected.