acording pathcnotes aden reconstruction donation: new diversified headwear and pendants inside “a treasure boxâ€; conditions of getting "hero's treasure box" have been partially altered: now you'll need more crystals to get a reward; elixirs are now impossible to get as a reward; new reward: "hero's attribute crystal" from where attribute crystal and attribute rune–super can be acquired. bolded part is not true or it works only for hero boxes who didnt give elixirs before anyways.
it is not a bug. elixirs are now impossible to get as a reward from treasure chest, not from hero's treasure box.