The biometric Face ID security in the iPhone X uses the cameras in the display's notch to check your face. So, it's quite surprising to hear that Apple is reportedly fixing Face ID by repairing phone's rear cameras.
This news comes from a report by 9to5Mac, which obtained "a new support document sent to retail stores and Authorized Service Providers" this past weekend. In the documentation, Apple formalized repair instructions for its Geniuses and third-party repair persons, instructing technicians to examine the rear cameras, as repairing those may solve Face ID woes.
While it seems incredibly confusing for one camera to be disrupting the other, commenters on the r/iPhone subreddit described a similar story back in November. Specifically, redditor Razerx1 stated that "the Genius Bar guy ... said that the cameras are connected," in that the "telephoto is connected to the true depth" camera, the front-facing sensor used by Face ID.
The Redditor sought out support when both the rear camera and Face ID stopped working, though certain camera functions, such as "pano, slow-mo, and time-lapse" were still working. When they tried to use Face ID, an error stating "Face ID is not available, try again later" appeared.
If the front and rear cameras are connected and dependent upon each other in the way this report suggests, repairing the back camera certainly makes sense for Apple. If Face ID can be saved in this way, it saves the company plenty of money, as the next step is to replace the entire $1,000 (or more) phone altogether.
Hopefully, Apple's learning from whatever glitch is causing this to happen, as a previous report claims all of the new iPhones for 2018 will feature Face ID, which will continue to replace Touch ID.