Oh thanks I will check that out the wrong text! Probaly just forgot to change it.
If you refering to the chests in the mermaid tower and i think one other location, those are already open by default, they do not contain anything, they are just for decoration. I think those chest also say they are empty then interacted with.
I will check out the skill, thanks for noticing!
Its not a bug the boss is actually immune to the dmg type you do, you are supposed to use the skill that changes your attack element, this is just badly communicated. I will add some text in the fight to give a hint.