Well then maybe I have a defective unit. Because face ID only works %100 of the time when upright. Otherwise it's %50
The phone doesn’t really care that ur lying down. Laying flat on ur back is basically the same thing as standing up. So if it works 100% of the time when ur standing, and only 50% when lying down, and for both positions u are holding the phone at basically the same spot to read ur face, then I would agree something is wrong with the hardware. Maybe something with the gyroscope.