Rear camera works intermittently

I would definitely think camera is the issue. Even though the camera module and the flash module are separate, they are electrically intertwined. This is the case on every iPhone schematic I have examined. I am not 100% on the exact process, because it's hard to know order of operations.

In short, an enable signal for the flash is transmitted by/through the camera module. So non functional camera usually means non functional flash. depending on the failure.

I see this issue enough it probably warrants some more detailed explanation. I think the thing that is easy to overlook in cases like this is that electrical interconnection is not easy to figure out with your eyes. The flash and the camera are independent, but they still talk to a lot of the same components on the logic board.

Caught in this web is the CPU, the Camera Power Management chip, the main power management chip and the strobe drivers. As well as the camera an flash/strobe drivers. It's hard to know exactly order of operations from the schematic alone, I'm looking at the datasheet for the LED driver right now because there are still bits of the process I don't totally understand (like why specifically the flashlight/torch function doesn't work if the camera is not functioning).

This chip specifically has a dedicated pin for flashlight function. But it's possible there are some checks in place, or something in the circuitry requires the camera to be present/functional for that line to work as well. Or Apple just decided to not use that PIN and made some workaround. I haven't had coffee yet this morning so I will probably have to stare at this a bit longer. But I'll update if I figure it out.