A reactivated built-in Administrator account will be automatically deactivated when upgrading from Windows 10 version 1803 to Windows 10 version 1809 according to a post published on Microsoft's
Japanese Technet blog.
Moreover, the issue arises only under certain conditions, when the built-in Administrator account is active, and there are other accounts on the machine which also have Administrator permissions.
A patch to be released during late January
"We are currently working on solving this problem, and at the moment we aim to release a patch in late January 2019," says Microsoft. "Therefore, if you need to upgrade before the patch is released, please confirm that you can surely sign in with a user with administrative privileges other than the built-in Administrator, before you upgrade."
It's important to mention that, although this issue will lead to the deactivation of the built-in Windows 10 Administrator, this account should not be used on a daily basis given that all Microsoft operating systems have it disabled by default since Windows 7.
Given that you will not receive any User Account Control (UAC) prompts while using this account and it will run with full admin permissions, any changes you or any program with elevated privileges make to the system will be applied immediately.
Disabled Windows 10 built-in Administrator account
Built-in local Administrator should be disabled for increased security
It's recommended to use a non-Administrator account to limit the potential damage malware would be able to inflict in the event of infection to harden the security of your Windows 10 installation.
"Also, if you have already upgraded and you have invalidated the built-in Administrator, you are signed in as a user with administrator privileges other than the built-in Administrator, activate the built-in Administrator," also says the Technet post.
This issue follows a whole slew of other quite 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.
