I think at least part of the reason Zen has a high win rate is because at lower levels he's actually quite a good healer.

Mercy requires some level of positioning, Moira and Lucio require some level of coordination and Ana requires aim. But with Zen just pop an orb on your Junkrat or Reaper and as long as their in line of sight you don't really have to risk your life to heal.

Combine this with the fact that lower rank games are inherently slower an orb of discord tends to force coordination even it's with selfish intent.

I think they should maybe look to a slight nerf in his ultimate with the goal of making his overall healing in a game less.