Versy Ah, that makes sense. If the Wind parameter of the Physics Constraint is used to animate the bones, then changing the scaleX of the Skeleton will move them in the same direction, which is the expected behavior.
Similar results can be seen in the Spine editor. (Setting the skeleton's scaleX to -1 via code is technically different from setting the root bone's scale X to -1, since everything can be scaled, regardless of the inheritance settings for each bone):
Instead of changing the scale of the skeleton, it would be better to flip the character by changing the scale of the actor. (I think it should work, but I haven't had time to test it yet.)