The healing only occurs until Undead Rebirth. While in Undead Rebirth, you can still take damage. Not getting below 1HP in that stage of it is irrelevant - because it's going to expire shortly after and you've already healed yourself. So you may as well pretend that the invuln expired altogether. It would be better if Undead Rebirth turned it into a full invuln like Hallowed Ground.