I have tested this skill with and without any morphes and sadly the skill does not stun the enemy each time I use it. It seems rather random how the enemy is stunned.
Just how I experience it.
I use the skill, the damage is applied but my enemy is not stunned (normal random mob, no immun boss). I use it a second time and it stuns the enemy (like it should), at the 3rd use it may or may not stun the enemy.
Normally I would guess from the skill description that the skill would always STUN an enemy for 2 seconds (100% chance to apply) and then the enemy is dazed for another 8 seconds (breaks on damage)
If the skil stuns it will work correctly and daze the enemy for 8 seconds.
However the initial 2 second stun is not working always.
Maybe you can have a look into this