I have read poop eating behavior may be learned. If Mom got the chance to eat the poop (to clean up the bed) and the pups witness that they think that is what they are supposed to do. (ick)
A breeder that cleans up after pups quickly will lesson the chances of this problem. Not an easy thing to accomplish as much as puppies poop huh?
It probably has something to do with how yummy the poop smells and tastes too. (double ick)
I think I would take her bed away and replace it with towels or small blankets that are easy to wash daily. Make sure she is eating stuff that only tastes good going in, not coming out. If she has always been a poop eater then she is probably taking it back to bed because she sees it as burying her treat. Lovely. Did I say ick?
Can you get her to potty on command? Shelby poops and pees before bedtime but since she sleeps with me pooping in the bed is not acceptable
good luck, this is a tough problem.