The problem is changing animation frames causes the physics body to be destroyed and a new one created in its place, and doing that prevents forces and joints from working normally. I can't explain why this is, it seems to be a result of the box2D physics engine, and I don't know what we could change to prevent it.
Don't animate or resize any physics objects - if you need to, use invisible static objects with the physics behavior and place any animated/resizing objects on top of it. This is mentioned in the first physics tutorial.
Ashley I implemened the technique you mentioned just fine, once I knew the limitation. I cannot see this mentioned in the tutorial however. TBH it should be in the manual, but the tutorial would also be ok.