I may have discovered a bug with the latest update .2018. After installing the update, Windows Update now shows Defender Definitions as available updates. This did not occur with the .2016 version.
Also, with .2016 - when you click Windows Defender in Control Panel, you got a simple box saying that it is turned off. With .2018 - Windows Defender UI opens with Defender off. Obviously, .2018 update changed something.
Has anyone else reported this? You should look into this. Thanks.