Cannot log into my Google account with Smart Lock, username and password prompt keeps repeating
The solution here was that at some point I had disabled Safari which doesn't really uninstall it as it can't be removed but does remove the app icon from your home pages. It also seems to have other effects as cookies or however the Smart Lock app is saving state fails to work, and thus will continuously ask for you to log in. 

As of iOS 12, this setting is found: Settings > Screen Time > Content and Privacy Restrictions > Allowed Apps



After ensuring that I had enabled Safari, everything worked perfectly.