Maps (also treasure and mansiom maps) lose ID information when placed in chest. Called Map Unknown. 
Now when you take out the map to check in your inventory, it retrieves the data, but after closing the game the map that is placed back in a chest/container turns back to Unknow again. When you rename the map, the same happens, but the name itself sticks.

To reproduce, make map or use a treasure/mansion map and put it in a container, relog.