https://arstechnica.com/gadgets/2018/10/microsoft-fixes-october-update-file-deleting-bug-resumes-insider-testing/

Microsoft has figured out why the Windows 10 October 2018 Update deleted data from some systems and produced a fixed version. The severity of the bug caused the company to cease distribution of the update last week; the fixed version is now being distributed to Windows Insiders for testing, ahead of a resumption of the wider rollout.

Microsoft is advising anyone affected by the bug to contact support. The bug caused files to be deleted, meaning that the only guaranteed way of restoring them is to retrieve them from a backup (if you have one). However, undelete software can often recover recently deleted files, and Microsoft's support can apparently assist with this process at no cost for those bitten by the problem. In the meantime, the advice is to use affected PCs as little as possible; minimizing disk activity helps maximize the chance of a successful undeletion.