Yeah, the wording is just ambiguous, unfortunately. I don't see evidence of a bug from my interpretation, but they need to use more precise language on this point.

For what it's worth, the Japanese patch notes are a bit clearer on emphasizing what actually changed:
以下のアイテムを獲得できる宝箱が、24人アライアンス向けの宝箱からパーティごとに出現する宝箱に変更されます。
The treasure coffer from which the following items can be obtained has been changed from a treasure coffer for the 24-man alliance to a treasure coffer for each party.