Why do mother cats eat their kittens' poop?
After giving birth, mother cats develop a strong sense of crisis. If they nurse in one spot for too long, leaving behind too much of the kittens' scent (milk smell, feces smell, urine smell, etc.), they will start migrating with their kittens to another location.
The mother cat eats her kittens' feces, which is also related to eliminating odor. The mother cat eats her kittens' excrement not because she is clean, but mainly to eliminate odor. 
Therefore, free-roaming female cats, or female cats with a strong sense of crisis, or mature female cats will take the initiative to clean up their excrement.
A mother cat will lick her kittens clean, trying to remove their scent and excrement, either out of fear of predators or to protect them. This is known as "eating feces" to make things easier for the owner.
However, some cats are taken away and raised by people before they are weaned. Even if they become pregnant as adults, their minds are still in the nascent stage and they will not have the various skills that a real adult female cat has (or they have never had a mother cat licking them since they were kittens. Because they were weaned too early, they did not learn this skill, so they will naturally not lick the kittens' feces).
Another reason is its absolute familiarity with and acceptance of its surroundings. Because in its mind, you are not its owner, but just a live old female cat. It knows that the shelter you provide is absolutely safe, the food you give is absolutely safe, and you will never attack it. Therefore, it is not worried at all that the strong smell will cause trouble, and naturally it will not actively lick the kitten's excrement.
