I think BHE's passive stun works correctly. It invisibly stuns forever.. or until BHE is dead or has his passive disabled. Disabling his passive ain't always easy even with BHE vs BHE which are both capable of disabling the other -- but with Moonchild.. BHE can't use his power ability to heal.. and it's that which causes passive disable to be inflicted on opponents.
What gets me more is that Moonchild is NOT "immune to all buffs" he simply doesn't SHOW any buffs. Matches would take half the time if allies were NOT invisibly granting Moonchild Heal Shield or Life Seal or Spell vamp... and he heals when he's hit. Which happens all the time. You can watch him heal.. when with no buffs and no vampire.. he shouldn't ever do when he's hit.