The bug

After taking a treasure or explorer map in your inventory the hover text of the item should show information about it (might not be the case in the trading GUI: MC-118141).
However, if you place them in a container block (e.g. chest), then reopen the world and open the container the maps show again only "Unknown Map".

How to reproduce

1. Obtain a treasure or explorer map (e.g. by trading) and place it in your inventory
   The item tooltip should now display information about it
2. Place the map in a chest
3. Close and reopen the world
4. Open the chest and hover over the map
   It shows "Unknown Map" again