The problem is that the camera module is tied to each board.
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I can confirm this. Pucchaced an At&t pixel 4 on ebay that wasn't working. And another unlocked version with a broken screen. Now the damage to the screen was at the top close to the modules. So when doing the board swap I decided not to swap camera modules because the fear of having a damaged module. After swap face unlock work before reboot. After that it never worked on both phones. Had to swap over the camera modules to get face unlock. Works like a charm now..