Opposing hero still immune after Mal'Ganis died

Today, I observed a bug regarding Mal’Ganis:

  1. Opponent played Mal’Ganis, so the opposing hero was immune in my turn.
  2. I played Sickly Grimewalker and Darkthorn-Quilter → Darkthorn-Quilter gained “Poisonous”.
  3. At the end of my turn, Darkthorn-Quilter was dealing it’s damage hitting and destroying Mal’Ganis.
  4. The immune spent by Mal’Ganis did not end after Mal’Ganis died
    5. All damage by Darkthorn-Quilter to the opposing hero was ignored, because the opposing hero was still immune after Mal’Ganis was dead.

Did they remain immune for the rest of the game ? Or only untiil your deathrattle finished dealing its damage ?

The Turtle isn’t dead until after your entire ability resolves so if they lost immune when it went back to their turn working as intended