I was looting a sunken ship, the upside down kind, when I went to loot the map chest. I found a buried treasure map inside, but when I put it in my hand, it didn't show the map in my hand like normal, it showed the sprite of the normal map texture. I decided to right click and the map just turned into a normal 0 : 4 scale map, with no "X" on it. I was experiencing heavy lag at the time. I have included a picture of my f3 screen with the treasure map in hand.

Edit:  It has happened again. More Info now. This happened last time, too. When I tried to open the map chest in a sunken ship, it would not open. I would right click it multiple times (that is my interact button, I checked) and nothing would happen. For some reason, the mobs around me did not move, at all. These 2 times, I decided to leave the world, and come back. Anything I did after right clicking the chest did not save, I was sent back to when the right click happened. When I right click again, the chest opens, and the mobs do not freeze. I moused over the Buried Treasure Map, and instead of having the used map texture, it had the texture of a un-used map. I am not sure about this, but I think that Buried Treasure Maps have a blue color on their item name (like this). If that is true, then strangely, the Buried Treasure Map's name was just "Buried Treasure Map". It was in Italics.

Edit 2: Small info, but info is info. When I right clicked the Buried Treasure Map, the name just turned into "Map".