The duplicate of MCPE-41898, but a bit superseding:

In the latest preview, even though you have exported the .mcworld to a folder that is not related to OneDrive, the save file is still broken and empty.

I post this to let mods know that this bug reappears in the latest preview of MCBE to attract your attention on this serious bug, so excuse me for my improper behavior. Plus, I've posted a reply in the bug report at the first line of this post.
Note: The world exporting was done in immediate miliseconds.

P.S.The directory in the snapshot is not a subfolder of OneDrive.