Face ID not working only when the phone is assembled

Hello everybody,

I’m a newbie DIY iphone repairer that learn everything from the internet. Recently I changed my iphone XR’s housing and screen because both my old housing and screen were damaged.

And I was faced with a mysterious problem. So before I clasped closed my phone’s housing and screen together I tested the Face ID and confirmed it was working flawlessly. But once I closed it the Face ID won’t work anymore (from Face ID to registering new Face ID).

A couple of days ago I managed to work Face ID with the phone closed only later to realize the screen wasn’t completed shut closed on its top right part.

When I used Face ID in that condition I noticed two red lights coming from the very left and very right from the turedepth system. But when I pushed and closed my screen completely as I was using Face ID, the red light from the very right of the system disappeared.

Looking at the truedepth camera system image above, I’m assuming the right red light came from dot projector, and therefore something is blocking it with the screen closed. Do you guys have any idea what could be wrong?

I screwed everything inside the phone. I hope I don’t have to tear apart the phone again to fix this since doing that takes a lot of time and energy. Seeing that Face ID works every time before the phone is sealed, I’m guessing there’s nothing wrong with all the sensors in truedepth camera system. If you got an idea that could help or you had a similar experience, please let me know below in comments!

Lots of Thanks,

Bo