Smart Lock unreliable/not working with NFC tag?
This is a known issue, https://productforums.google.com/forum/#!topic/phone-by-google/DyxZwJpsOwY &amp; https://issuetracker.google.com/issues/62268615. 

In addition, the NFC Smart Lock feature has been removed from 7.1.2 (I think that was it) and is still gone in Android 8.0.0 Oreo.