Light breeds 12-20 weeks of age confined housing 5.0 square feet per bird.
Bantams same age band 3.5 square feet per bird.
You had 23 birds in that 64 square foot coop or 2.8 square feet per bird.
Birds get stressed from the confinement.
Let's go over what you told us, you heard a bird scream but there was no alert.
"I did hear one scream a little earlier but I was holding my peachick and thought it may just be a fight among them and couldn't check and then heard no alert ... "
This indicates that it likely wasn't a predator because why would the flock alert on their actions.
The chicken is missing a lot of feathers and you also mentioned that you "thought it may just be a fight among them" and that the attack happened in daylight, it is rare for rats to be out and about then (they also will gnaw on just about any area they can reach on a bird), also weasels are dusk to dawn critters (and while not unknown to eat a bird from the hind end are more apt to go for the neck and a blood letting), raccoons also unless sick or very hungry are also not usually out at that time (they also need more of an opening to get through).
I've seen the results of chickens doing in another chicken. Once a bird goes down the others are more than happy to join in.
The end result is a dead chicken with a hole in it and its intestines at the very least severed, the chicken then dies. After that those little omnivores go to work.
To err on the side of caution, you need to beef up your setup which you are doing, setup some traps that can hold critters from weasels to raccoons in size, and think about going after rats and downsizing your flock (you do have plans for this) or increasing the size of your coops.
I've got to do the flock size decrease shortly or I'm going to have some chicken eating chickens as well.