I have the same problem on Mi8, it works with almost every face. The same issue showed a few ROMs ago(weekly) and is still there.
My brother has global stable version(10.2.2.0) on Mi8 and faceId works fine.
Anyone else has problems with downloading update via OTA(Mi 8)? When I click check for updates, I see there is an update, but when I click Download, Toast shows up and says that my MIUI version is up to date.