I tried saving a default terrain in both 1.2 and 1.3 and comparing the jsons, and yeah, the 1.3 terrain coordinates save even when you don't sculpt it, and of course the 1.2 terrain doesn't save coordinate info. It looks like it needs to have a way to assume if there's no coordinates in the file, it was made in 1.2, and load the default shape like it used to.