The way I see it, I also think that it's a MIUI problem. Probably the way they handle there security for locked phone. But this is a hypothesis based on my limited knowledge of MIUI and Android. Ingbrzy has more experience with it so I think that he'd know better.
I also have the issue for trusted bluetooth devices which should unlock the phone when connected. I've added my car's radio and it's not working. This is also a Google feature and the menu is only accessible when paring a new device. Xiaomi also has this feature in the security menu but it apparently only work with Xiaomi devices such as the Mi Band 3.
Edit: After digging around a bit, it seems that the Google Smart Lock isn't supported on MIUI even though it can be found in Settings -> Additional Settings -> Privacy -> Trust Agents. This seems to be the module that unlocks the phone when using Voice Match or Trusted Devices.
Envoyé de mon MI 8 en utilisant Tapatalk