Bug ConfigurableJoint doesn't move straight to targetPosition

I'm not sure if this is a bug.

Lets say I have a cube that I want to move to a new targetPosition via a ConfigurableJoint. That new targetPosition is marked by a sphere. The position of the connectedBody is marked by a cross.

The moment I set the targetPosition, you'd think that the cube moves along the green line towards the sphere, but what actually happens is that it drifts down a bit and comes back up. It always seems to drift a bit in the direction of the connectedBody and suddenly remembers that it actually wants to get to targetPosition.

How come?

 

I've done some experiments that show that the targetPosition may not even be reached. I attached a Package that shows that.

 

And here a screen capture of the problem:

Excerpt from the readme:

 

It appears that enabling swapBodies works as a workaround and fixes the problem. I don't get why but I take it.