For me, this was caused by the map files being in the wrong format, along with issues in the landed_titles file. Make sure your files are saved correctly.
http://www.ckiiwiki.com/Map_modding
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While I'm here, I'm having a few issues I can't seem to solve. Here are pictures of what I mean.
First, province names seem to be missing or in the wrong places when picking a ruler. I've been told it's due to cultural names, but removing them does nothing.
http://steamcommunity.com/sharedfiles/filedetails/?id=948192661
Second, how can I disable this DLC so that the monuments are showing up like this?
http://steamcommunity.com/sharedfiles/filedetails/?id=948194790
Last, some provinces, like the Canaries there, show up black when zoomed out. It also happens with other islands. Does anyone know why?
http://steamcommunity.com/sharedfiles/filedetails/?id=948194149