When I open a chest, I notice that the lid goes up, but at the same time, the chest looks closed, albeit with a lid sticking out of it. I did have chests stacked on each other when I observed this, and it was a double chest, backed against a wall. I played around a bit, and found that when breaking a double chest, the breaking pattern appears on both halves of the chest, although only the half you are mining breaks. And if it's a stacked double chest, a strange graphical remnant of the half you broke will remain, and when you break the other half, there will be a ghost image of a double chest where the one you broke was. I fiddled around a bit, and couldn't reproduce the bug at all, so I took some pictures, and attached them. Hope you can figure out what's happening.