What's up with this weird Chest of the Damned?

So, TLDR: I just finished doing Old Brinestone Fortress and Ancient Gold Fortress in that order. I got the treasuries and storage room looted, with that done I've sailed back to Plunder Outpost to sell all my treasure at the Sovereigns Outpost where I see... this odd chest.

I'm guessing it's a bug but has anyone seen this before? Really odd sight to see.