In the old flow, navigate to chrome:settings/lockScreen, make sure SmartLock is enabled, click the 'Turn Off' button, wait a few minutes, see the UI change back to enabled without further intervention (notification is shown saying 'Smart Lock is enabled for this device' and I get an email saying Smart Lock is enabled.

I'm seeing this in M71, but I'm going to go back and check 70 as well. It may be a bug in the backend though.