I have some ocean explorer maps which were purchased from a cartographer in the previous gave version. These maps used to point to an ocean monument in the second map ever created. After old world sync into the new game, these maps now point to an ocean monument in what would be the 10th map. This is in a world which already has the previous 9 maps fully explored to complete former game version map room achievement. Maybe this is part of the conversion process, don't know.