After logging off and back on, my copied worlds are disappearing (this has happened twice now). This seems to be tied to worlds where the world name is the default "Copy of [name]", because those are the ones that disappeared.

11-27-17 EDIT: I conducted an experiment by trying to recreate this glitch. What I found was disheartening, but perhaps avoidable. First, I created a new world, made a copy of it, and exited the game. After loading it back up, the new world and its copy were still there. Then, I made a new copy of my main world, and again exited and relaunched the game. This time, the copy was gone. Here is a link to a video I shot of this occurring. I don't know if my world is corrupted somehow, but if that's all it is, I suppose I can redo everything and hope this doesn't happen again.

11-28-17 EDIT: This happened again on my new world, and I believe I know (at least partly) what causes it. I created a copy of the world, deleted it, made another copy, deleted it, and so on. I did this a few times before I thought to check to see if the glitch occurred, which it did. So, it seems like this bug is caused by creating multiple copies and deleting them.