Expected behaviour: Map marker does not vanish on world reload on any zoom level map.

Current behaviour: Banner map markers vanish after reloading the save file on fully zoomed out maps (zoom level 4).

Marker can be applied to the zoom level 4 map normally, but after "Save and Quit to Title" and then reloading the world, the marker has vanished. Affects multiplayer (vanilla server.jar) too.

Reproduce:

1. Create a map (right click an empty map).
2. Zoom out the map fully (zoom level 4).
3. Right click a banner with the map. The map now has a banner marker.
4. Press "Save and Quit to Title".
5. Reload the world.
6. The map marker has now vanished.