iPhone turning on with black screen - backlight is on

Alright fellow repairers, here’s a tough one :

I repaired a broken screen on a completely destroyed iPhone 8 (both front and back glass smashed) - but the phone was still working perfectly. I made a chassis replacement, successfully. Then, I changed the screen, successfully also as usual.

I let the phone charge (battery at 85% capacity), and come back an hour or so later : big surprise, the phone won’t turn on. I then start to disassemble it, and notice that the backlight of the phone is on ! The buttons are working, the phone even responds to touch : I could turn it off by sliding the power slider (blindly tho).

Any idea of what could’ve go wrong there ? I tried another screen (Apple), the original screen (which was crappy af, had already been replaced for sure), but none worked. Even a DFU restore won’t help.

Has any of you ever encountered that type of problem ? Is there a diagnosis that I can make to help ? I’m starting to have some micro soldering tools, can do some measurements for sure.