[cs6] Curved motion tween bug ?

Here's what I'm trying to achieve : move start frame of a motion tween (make it stay still for a few frames).

The way it works :

- put a clip on screen

- create tween, move clip on last frame to create linear motion

- hold and drag first frame so the clip will stay still for a while : works

- curve motion path : still works

Now for the buggy way :

- put a clip on screen

- create tween, move clip on last frame to create linear motion

- curve motion path

- hold and drag first frame so the clip will stay still for a while : clip will bounce to the position it was previously on that frame  !

See the dots in mid air ?!!

Any idea ?

Thanks !

PJ