Copying then zooming out an Explorer Map make it lose its structure pointer

To reproduce:
1. Buy an Explorer Map from a cartographer
2. Make a copy with an empty map => Both maps works but have lose their special name/color
3. Zoom out one of the copy with paper => The unzoomed one has lost its structure pointer

This bug doesn't happen when zooming out an original Explorer Map (and it keeps its special name/color)