I’ve noticed that some Minecraft worlds no longer display when you open Minecraft if their world folder has been manually placed inside the minecraftWorlds folder. Some worlds still display when using this method. I thought it was a world size issue, but I can’t narrow it down exactly. There was previously no issue backing up worlds this way. I know there’s nothing wrong with the world in the folder and it’s happened with several different copies.
Please provide the affected world. If it's too large to attach to this report, you can upload it to a file-sharing site like OneDrive, Google Drive, or Dropbox, and then share the link.