It's not a huge bug or anything, but the debuff is only triggered when you move before you cast a spell. Meaning you if you stand still and cast a spell and start moving 0.2 sec later you will not get a debuff, it can be a little anoying at first, but once you get used to it, you will no longer be perma slowed.
Only downside is that this doesn't work with channeled spells (mostly affliction), as for some reason it still gives you the debuff half way the cast. Enjoy!