As shown in the video.
On immobilize you get and instant power lash proc, while the target is not offbalance yet. 2nd whip puts the target offbalance (should be first whip, 2nd whip should be the powerlash)
On stun you get offbalance of first whip, and powerlash on second whip.
Tooltip ingame states it should be as it is on stun. The immobilize one seems like a bug.
Normally all of this is not a big issue. But when using sets like Stuhns Favor it is rather annoying.
https://www.youtube.com/watch?v=jm_kV1Lfc1s&feature=youtu.be
Why do I even try