I've had a very similar / weird experience over the past week...
So, I needed to replace a broken screen. I did that and I noticed a couple of days later that the flashlight wouldn't turn on in the Control Center. Also, when I take a photo with the flash set to "On" it does not fire properly... The bottom yellow / amber flash fires but the bright white flash does not... I tried reinstalling the screen several times but no go...
Several days later the screen started to act up (touch was misbehaving) so I managed to get a replacement. This time, I was real careful, I hooked up the 2 screen ribbon cables (touch + main LCD) and turned it on. Booooom the LED fired right up in Control Center. Awesome, I built up the rest of the screen and assembled the phone as normal, turned it on, NO FLASHLIGHT. Shiiiiiiit tried all sorts, nothing worked so I gave up.
A week after that, the replacement screen was also began to act up - same touch issue. So I bought another screen from another seller... Tested the flashlight again and it worked fine. I built the screen / phone back together and the same issue again, the flashlight WILL NOT WORK. I'm usually pretty good with these things, I've repaired dozens of phones, laptops etc. and I cannot work this one out.
Is it possible I am shorting and killing something in the LCD when I've reassembled the phone? All 3 screens (2 of which have touch issues anyway) will not activate the flashlight? But I know at least 2 if not all 3 of them did fire up the flash prior to rebuilding the phone?
I've tried connecting the bare screens again with no earpiece / proximity sensor ribbon connected... I've removed the camera... I've done soft / hard resets, new phone restore, back up restore - nothing makes any difference... no flashlight.
So, it's either something I keep damaging in the screen with each replacement or it's a very weird iOS bug?
Any advice / help will be hugely appreciated - thanks in advance!