Hmmm - my Dad was in the Pacific theater, and spent time on several islands, including New Guinea. He mentioned encountering headhunters, but not cannibals.
And the headhunters had been trained - bribed, perhaps - to rescue any downed or lost Allied airmen they encountered. Quite a few were returned to Allied military bases, and the natives who did that were rewarded.
They were free to do whatever they wanted to Japs.