Kuva Liches becoming Invulnerable

[Kai]

So I had this encounter in a Sabotage Mission with a full squad of random players. The moment I spawned in the mission, my lich has already spawned and a random player was able to somehow perform a mercy on them, of course this did not kill it and its HP bar became Gray. It can still replenish shields, but upon depleting it - nothing. No prompts for further mercy, you can just keep damaging its shield but nothing else.