I had to cook each cannibal piece on the fire before eating it to unlock the achievement.
I tried making 24 drying racks and put one cannibal on each rack (a set of legs and arms on each). After I had 24 cannibals dried, I went around to all the racks and ate each set. Unfortunately, the achievement did not unlock.
Next, I started cooking and eating fresh arms and legs from cannibals, and the achievement unlocked after four or so eaten cannibals. I had eaten some cannibals from the fire prior to trying this.