I can't open your capx right now but when the animation frame is changed the collision shape is recreated. The only way for it to know if the collision polygons of two different frames are the same is to compare all the points, which may be slower than just recreating it. Also note changing size also causes the collision shape to recreated as well.
So to the physics engine it's a new shape, so it causes a new collision. I guess a workaround in the plugin would be to modify the collision shape instead of recreating it, but there are many caveats with that, and that's something that's absent from the chipmunk documentation so it's not really something it was designed to do.
Anyways the solution is to not change the frame on physics objects. If you want to change appearance, use a seperate object.