Mystic slow movement speed debuff bugged?

[Dr. Nuke]

I'm testing Mystic Slow on Warden in sandbox mode. Whenever I deal spirit damage, Mystic Slow doesn't seem to debuff enemy movement speed at all. I thought it could be a cool combo with other items, like Mystic Shot. But when I hit someone with Mystic Shot + Mystic Slow, my target doesn't lose any movement speed at all, neither from any other source of spirit damage. The item seems complete garbage if this is bugged?