The spell is listed with the tooltip as follows:
“You disperse into pure Shadow energy, reducing all damage taken by 90%. You are unable to attack or cast spells, but you regenerate 6% mana every 1 sec for 6 sec. Dispersion can be cast while stunned, feared or silenced and clears all snare and movement impairing effects when cast, and makes you immune to them while dispersed.”
The way this is written sounds like while dispersed you are immune to these CC effects. However, there have been multiple times since prepatch where I use this ability in PvP, often to break a stun, just to find myself stunned AGAIN while dispersed. I can only figure this must be a bug.