Apple replacing some iPhone X units plagued with Face ID issues
And now, according to another leaked document sent out by Apple to its Authorized Service Providers, the company has detailed the process the service staff should follow when dealing with iPhone X units plagued by Face ID issues.
Apparently (and somewhat counterintuitively), the Face ID problem has something to do with the rear camera on the phone, as the document says the first step should be to run a diagnostic test to see if Face ID issues can be resolved with the repair of the rear camera.
If that fails to solve the problem, the staff can initiate the replacement process, offering affected users to exchange their unit for a new one.
As for the relation between the rear camera and Face ID, while there's no official information, an iPhone X user shared on Reddit a few months ago that a Genius Bar staff had confirmed to them that the True Depth camera and telephoto lens are connected.
You can read the complete discussion by heading to the Reddit link below.