Prior to re-releasing the October 2018 Update our engineering investigation determined that a very small number of users lost files during the October 2018 Update.  This occurred if Known Folder Redirection (KFR) had been previously enabled, but files remain in the original “old” folder location vs being moved to the new, redirected location.  KFR is the process of redirecting the known folders of Windows including Desktop, Documents, Pictures, Screenshots, Videos, Camera Roll, etc. from the default folder location, c:\users\username\<folder name>, to a new folder location. In previous feedback from the Windows 10 April 2018 Update, users with KFR reported an extra, empty copy of Known Folders on their device.  Based on feedback from users, we introduced code in the October 2018 Update to remove these empty, duplicate known folders.  That change, combined with another change to the update construction sequence, resulted in the deletion of the original “old” folder locations and their content, leaving only the new “active” folder intact.  Accordingly, below are the issues we have identified and fixed:
Using KFR the user redirected a known folder to a different drive. For example, suppose you ran out of space on your C drive. You want to save some files separate from your primary folder, so you add another drive to your system for these.  You create “D:\documents” and change the location of the files known folder from the original “old” location c:\users\username\documents to D:\documents.  In some cases, if the contents of c:\users\username\documents were not moved to D:\documents, then a user could also encounter this issue.  When the October 2018 Update was installed the original “old” folder was deleted including the files in that folder (in this example c:\users\username\documents would be deleted; d:\documents, the new location, would be preserved).
The user configured one or more of their Known Folders (Desktop, Documents, Pictures, Screenshots, Videos, Camera Roll, etc.) to be redirected (KFR) to another folder on OneDrive.  For example, the user changed the location property of the documents folder from c:\users\username\documents to another folder.  During this process the system prompts the user and asks if they would like to move the files to the new location.  If the files were not moved and the October 2018 Update is installed the original “old” folder was deleted including the files in that folder.
The user used an early version of the OneDrive client and used the OneDrive settings to turn on the Auto save feature.  This feature turned on KFR for the Documents and/or Pictures folders based on the user’s choice but did not move the existing files from the original “old” location to the new location.  For example, if a user turned on Auto Save for pictures the location of the Pictures folder would be changed from c:\users\username\pictures to c:\users\username\onedrive\pictures, but no files would be moved.  The current version of this feature moves the files. If the files were not moved and the October 2018 Update was installed the original “old” folder was deleted including the files in that folder (in this example c:\users\username\pictures would be deleted; c:\users\username\onedrive\pictures, the new location, would be preserved).
We have fully investigated these issues and developed solutions that resolve all three of these scenarios, so the “original” old folder location and its contents remain intact.