"The TrueDepth camera system incorporates tamper-detection features. If tampering is detected, the system may be disabled for safety reasons."

https://support.apple.com/en-sg/HT208108

It's official now, Apple acknowledges that the Face ID system is boobytrapped.

Don't know how exactly, however it may be due to security that the data does not get stolen, or safety, that exposed IR beam power stays below regulation limit when the device has been disassembled, or Apple just wants to mess with people.