Basically after dying in hardcore mode, "Delete world" button doesn't do exactly what it should, it just kicks you out to main menu with bugged background, and after pressing ESC button on keyboard it kicks you back into the game, so the only way to leave the world is to start spectating and then leave the world by pressing ESC again and pressing "Save and Quit to Title".
I've recorded video of that, quality is poor due to 10 mb limit in here. But it's enough.