Elder slow bugged?

I could be Mistaken but all searches online seem to say the elders slow is only ment to apply while within the aoe?
Been doing the fight a fair bit and it is slowing me well after i have left the aoe, if im behind him or even on the other side of the arena

I don't know how it's supposed to work, but I believe it's always worked the way you described it.

Yup, it's always been "global". People used to wear Kaom's Roots for this fight.