Lore wise, technically isn’t the first time you defeat a boss the first time they’ve ever seen you? When you die the world resets, hence why they do their little intro bit each time. From their point of view you suddenly turn up out of nowhere and beat the shit out of them. They have no knowledge of the 80 previous versions of you that died.

Actually would be kind of a cool concept for a game. A rougelike where you kill the hero as many times as you can before you die or something.