It's as I suspected. Unity and Box2D weren't designed to work the way you were attempting.
If the colliders move independently, they shouldn't be part of a composite collider.
If they don't move independently, you can ignore the warning BoneFollower gives you.
But if they do move independently, you'll pay the price of a costly physics body rebuild every frame. Or it will work unpredictably. Or it won't work at all.
That's just how Unity is designed.