I created Map17 inceased the zoom to level 3 and then cloned it.

Then I lock one of the maps with a glasspane.

(The map is empty, as I didn't hold it in my hands yet - but that's not the issue it also happens when the map is partially discovered)

Then I start updating the second map until it's fully/partially discovered.

Then I lock the second map, the map now is empty like the first map that I have locked.