The October World Boss (Dark Beast Alpha) does not drop purple box rewards at all, apparently [Slayer's Cache of the Dark Beast Alpha]. One sometimes gets a purple box inside a blue box, which is a similar mechanic from other events that dropped purple boxes by themselves as well, on separate occasions. However, the blue boxes also apparently incorrectly drop [Slayer's Cache of the Inferno Lord]. It has been noted that the other "new" World Boss did something similar, so it looks like the loot tables are bugged?