Named maps in containers show as 'Unknown Map'

I name my maps relative to the spawn map, so "N1" for the map one north of spawn, etc. All of them would have been created in version 1.4.x. Survival world, playing single-player, never been in the beta program.

In my Ender Chest, they all show up as "Unknown Map". When I pull them out, the name is restored and all explored areas show as normal, so this is a minor bug.

Updated by Oval: To reproduce:

1. Load a map, put it in an enderchest.
2. Reload the world.
3. Open the enderchest and hover over the map.
   → Map displays as 'Unknown Map'