Lots of fascinating tidbits here, if teardowns are your thing. But deep down, all the way in Step 10:

Face ID works even when we disconnected the front sensor assembly. However, any display replacement knocks out Face ID. We tried transferring the sensors from the old display and porting over the Face ID hardware, but no dice. It looks like the display is serial-locked to the phone.

And:

TL;DR: Unless Apple revises this behavior in software, screen replacements outside Apple’s authorized repair lose all Face ID functionality.

Is this a security measure to prevent a replacement screen from overriding Face ID on a stolen or seized iPhone? I suspect we’ll never know the logic behind this decision until far down the road.