It was almost certainly a mistake, world edit isn't the most versatile tool and unfortunately that sort of stuff happens. If you lost super valuable stuff, I'd suggest reaching out to the Tahmas, otherwise just give it a few and do something else.
No, don't worry, I understand it was a mistake, and really it was my own fault for placing the chest in a not particularly easy to see place. Thanks for the suggestion to talk to Tahmas, though. I'll make sure to talk to him once I feel slightly less salty.