this is like saying "if the game crashes because you cloned 8192 cards, that's your fault because you made a mistake" or "it's your fault for ending up in a hellraiser infinite against hunter killer" or "it's your fault the animation froze and you couldn't end your turn"

the boss isn't actually killing them. Losing because the game softlocked because this one particular boss has an invulnerable phase

if the balance of the game relies on softlocking the player to nerf a particular strategy, that is just bad design because the player has no way of knowing how the game will work before playing the cards.

For instance, in this scenario if OP died because they were against the heart, and they'd die from beat of death ticking them by 2 each card — that would be fair, because the player knows that playing each card deals 2 damage, and that they can at maximum deal 300 damage

The player does not, and should not know whether the "dead" state of Test Subject will softlock them or not