Front Camera is Not Working After Screen Replacement

I run a small repair shop and have fixed over 10 screens for iPhone 6's and all have gone smoothly (except for one other one, but that was because I was shipped a faulty screen replacement) but I just ran into an issue while fixing a screen for one of my customers.

When I got the phone, the screen was completely shattered and wouldn't display anything, and the top right corner was just completely missing the glass. The frame was slightly bent on the bottom left side, and when I took the phone apart, I could tell that the screen had been repaired by a third party before, as the screen was not original and had the little stamps that a lot of aftermarket screen makers use to identify their product.

I disconnected the battery, then went along with the repair process the same way I always do, and made sure that I didn't mix up any of the screws or anything like that. The replacement screen I used didn't come with the small parts already installed, so I transferred them over to the new screen.

When I tried to check the camera to ensure the repair was successful, the rear camera would work, but when I switched it to the front camera the app froze on the blurred out grey screen that the iPhones show before switching cameras. I thought maybe I messed up the camera while transferring the flex cable since they are so sensitive, so I installed a brand new one, and still no luck.

I had her take the phone home, and update it to the newest firmware to see if that would help and it did not. Now instead of the grey blurred out screen like before, the phone displays an image like the camera took a picture with the back camera as soon as you switch it to the front camera.

What could possibly be causing this to happen? I made sure to clean everything with 99% isopropyl alcohol before connecting everything so there shouldn't be any oils left from my fingers or anything like that either.