for 2 separate maps, opening a treasure map just started filling in the map for the current chunk I was in, that clearly does not match the terrain of the map in question (in both cases I was basically in open ocean, just small islands). I had a locator map open in my off hand at the same time when I first opened the map. It was basically treating the map like a new blank locator map, instead of being fixed to the location where the treasure is.