A reactivated built-in Administrator account will be automatically deactivated when upgrading from Windows 10 to Windows 10 version 1809. Microsoft’s Japanese Technet blog says the issue arises only under certain conditions. A patch to be released during late January 2019, Microsoft says. The issue follows a whole slew of other serious problems faced by Microsoft’s Windows 10, with the operating system deactivating and downgrading Windows Pro licenses to Windows Home and breaking the process of changing file associations being just a couple of them.
Source: https://www.bleepingcomputer.com/news/microsoft/updating-to-windows-10-1809-deactivates-built-in-admin-account/