Slowing potency of debuffs on you

what is this supposed to apply to? there are a number of things it doesn't. one im experiencing right now is Spider Webs, says you are slowed, and sure enough im slowed even though i have over 100% reduced slowing potency of debuffs

spider web is a debuff that slows, and my reduced slowing potency of debuffs isn't applying to it. seems like a bug