The feature is officially available for Mi 8 only. We enabled it for all devices supporting Face Unlock. The funny thing is, Mi 8 EE is the ONLY device with an old implementation of face unlock, so I'm not surprised that it doesn't work right now. Hopefully, Xiaomi updates face unlock in next release.
I didn't say worse, I said old. All devices switched to a new API of face unlock, except Mi 8 EE. Why? Ask Xiaomi. I'm just stating the facts from observing the code.
Maybe they will update their code on the next release, maybe not. We have no idea. Mi 8 Pro seems identical to Mi 8 EE, yet Mi 8 Pro got the new implementation, Mi 8 EE did not.