Yes, cannibalism does occur in a flock, but it's more common that a predator of some sort is inflicting the damage. We can't tell you what it is because we aren't there. But you can sleuth around for clues.
What is your location? It will give us an idea of what sorts of wildlife you have there.
Is your coop predator proof? Is it rodent proof? Are there any cracks or gaps up high or very narrow, thin cracks down low? Rats can collapse their skeletal frames to fit through a crack that is a mere one inch wide. Rats have been know to eat the exposed flesh of chickens as they sleep helpless on their perches.
Owls can fly through a narrow vent high on the coop wall to gain access to the chickens as they sleep. Similarly, raccoons are very adept at finding narrow cracks in a coop to gain access. They will do that sort of damage. Weasels of all sorts can squeeze through cracks to enjoy eating parts of chickens as they are helpless in a coop at night.
You need to go over every square inch of your coop and run to find these tiny, nearly invisible entry points. Small mesh hardware cloth should be used to close off any cracks or gaps.