Windows 10 comes with a built-in Administrator account which is disabled by default during setup, and it seems Microsoft hopes to keep it that way.
According to a blog post by Microsoft Taiwan, upgrading from the April 2018 Update to the October 2018 update will deactivate the Administrator account if it has been previously activated.
This occurs during the upgrade process when there are already existing user accounts with Administrator privileges, but I expect this could cause issues if users delete such an account expecting the Administrator account to still be valid.
Microsoft says they aim to release a fix in late January 2019, but in the meantime suggests users have more than one valid account with Administrator privileges when they update to the October 2018 Update.
If you have already updated and prefer to have an active Administrator account, you can re-activate it in Computer Management >Local Users and Groups > Users.