Reproduction:
-Place a map inside a container. (shulker box, chest, etc.) .
-Leave and re-enter the world.
-Open said container that holds the map.

Result:
The map will be renamed as "Unknown Map" when briefly hovering over it.

NOTE:
If a copy of the map is in the player's inventory when re-entering the world, then the maps won't be re-named as "Unknown Map".