I was playing a co-op game with karnac where a dracolich raised an enemy mourner. The converted mourner, however, was giving its life boosts to the enemy side when we lost units, proven by their gate guard (then one of only two units controlled by the enemy, the other being a heretic) gaining two life when it killed two of my barracks-spawned soldiers.
EDIT: I do not have a dracolich and, so, am incapable of replicating this.
Thats a bug! Someone need to fix that.
We are looking into this, moving this topic to support.