This happened to me once. I don't know the reason. Only if you are offered a choice between the cloud version and the local copy, never pick the cloud copy even when it says it is newer. This problem has never been fixed. And for some reason they won't let us back up our games.
Try making a copy of your world periodically and always rename immediately. don't keep two world's with the same name.
Others might have more info, this is just what I have found so far.
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