I have seen your complaint before and still don't understand. Face ID on my phones(X and 12P) have always worked fine in the dark. Here is a description of how Face ID works in the dark:
"Face ID uses infrared to scan your face, so it works in low lighting conditions and in the dark. The TrueDepth camera also has what Apple calls a "Flood Illuminator," aka an infrared light that illuminates your face in the dark so the dot map and the infrared camera can do their jobs." According to that, visible light should have no bearing on the operation of Face ID.
The quote is from:
https://www.macrumors.com/2017/09/13/how-iphone-x-face-id-works/