Windows 10 comes with a built-in administrator account that is disabled by default during setup, and it seems that Microsoft is hoping to keep it that way.
According to a blog post by Microsoft Taiwan, the upgrade from the April 201
This happens during the upgrade process when there are already existing user accounts with administrator rights, but I expect this may cause problems if users delete such an account expecting the administrator account to remain valid.
Microsoft says they aim to release a solution by the end of January 2019, but meanwhile, suggests that users have more than one valid account with administrator rights when they update to the update in October 2018.  If you already have updated and preferring to have an active administrator account, you can re-enable it in Data Management> Local Users and Groups> Users .
Via Techdows, Borncity