I updated my world from 1.13.2, to versions 18w48a/b. I noticed some of my roads (made of gravel and dirt path) disappeared and were replaces with grass. Another thing that disappeared were some ravines. To reproduce this, get a world with a lot of roads and villages in 1.13.2, then update the world to those snapshots (BACK UP THE 1.13.2 VERSION!!!), then observe where the roads/ravines disappear.