Champions do not trigger map events triggered by champions unless they are the highest level unit in the stack. You have to leave the stack with your hero to initiate quests or pick up loot.
It behaves as if the trigger checks the highest unit in the stack for triggers. This is probably done so that loot picked up goes to that character. It needs to check if any champion is present in the stack, and then give loot to the highest level champion in the stack, even if it is not the highest level unit.