on a survival world (cheats off, no one else has joined, no data packs or anything), while exploring and filling in a map, I found a shipwreck floating high up midair, directly above another shipwreck, so I took a screenshot and moved on, but when I came back after filling in the map, it was just gone, and the shipwreck below changed, but it was still clearly the same one I had looted previously (chests exactly how I left them)

quick addition: the shipwreck that partially overwrote the one in the water, upon closer examination, is clearly a different structure from the one that was in the air