Hey, I've not long had the same issue, luckily I had a spare iPhone 6 on hand and swapped the rear cameras, no idea how I broke it as I never touched it.
I've been doing phone repairs for years as well.
And it's weird that the back camera causes the back flash not to work, considering they are now separate units and not on the rear camera itself, as soon as I replaced rear camera, both rear camera and flashlight worked!