Permanently losing boss loot post kill with no retry option feels terrible

At the end of act 1 I beat Count Geonor after several tries. Then I died to a dot or something after he did. I saw this good loot drop (presumably the best loot so far since it's act 1 end boss), I go back to the waypoint, loot is gone, no way to redo the fight. Was it a permanent buff item? Rare orbs?

This feels TERRIBLE and has me questioning whether I want to keep playing the game until fixed. Makes the whole 10 ish hours I spent on Act 1 feel marred / lesser and destroyed all satisfaction for killing the end of act boss. Really bad design. One of the most disheartening experiences I've had in a game. Please change this.