Glitch in flat maps

Every time I create a flat map its fine until I close out of the app and shut off my computer. Then when I get back on later the name of my map is blank and the seed number is changed to 0. When I open the map there is only a small portion around me that is still flat and there are huge cliffs around me creating a dome. When I fly above the cliffs there is another whole world of natural infinite land.