While it is extremely hard for the Insider Programme to give a true 360 of what Windows changes do - after all, the system in voluntary, and Microsoft has no control over what machines people are using with it, this is a massive blooper nonetheless and one that will take quite a bit of explaining.
The Wondows 10 update rolled out globally on October 2. It further said that the company is investigating the reports and the update will be republished when a fix is arranged to address the critical bug.
If you have manually downloaded the Windows 10 October 2018 Update installation media, please don't install it and wait until new media is available.
On Saturday Microsoft decided that the tide of consumer reports complaining about the latest Windows Update was high enough to pause the software rollout. If you were affected and call Microsoft, you may want to reference the support articles mentioned by Sarkar to see if that gets you anywhere. If your files are missing, you have been affected by the bug.
In theory, this means when a new Windows 10 feature update rolls out to the public, all of the major bugs should have been squashed. Here you will get the option to go back to the previous build.
Microsoft's Your Phone developers have acknowledged that this is not the intended behaviour, though we wonder how such as easily detected issue could have slipped through.