Servants effectively invulnerable if attached to heart with no walls
You can reproduce the bug by placing a heart, that ends up with ONLY servant coffins. Especially useful in DUO servers where you get two hearts.
Since there are no walls for that heart, you CANNOT put it into breach mode. Which means the servants do not lose health, and can be resurrected over and over. Making them basically impossible to stop if the person is online to revive them.
The underlying bug is not putting a heart into siege because the heart has no walls, and the exploit is attaching only servant coffins to it.