+Magic Skill damage does not buff Magic Arrow Magic damage

Found a +5% Magic Skill Damage Opalvein today and wondered, which of my four Magic damage users might benefit the most from it.

I found that Abyss Warlock, Bone Necro and Hammerdin all gain more damage from the +5% then a SoJ BUT they then also suffer a massive hit to the Mana pool. So i tried my Magic Arrow Zon last since her Mana usage is basically 0. However adding Opalveins +5% Magic Skill damage did absolutely nothing to her Magic Arrow damage. No change at all!

Magic Arrow skill converts a certain percentage of your physical damage into Magic damage, so you end up with a mix of physical and Magic damage and that Magic part should be raised by Opalvein’s +%. Yet it’s not.

So … WTF??