Interesting question - I have got the impression that "Muslin" is not the same everywhere. Here in UK it is loosely woven cotton fabric traditionally used for straining curds from whey or seeds and skins from fruit pulp in jelly making. I've seen some projects posted which are described as stitched on muslin but that looks much more like what we would call calico which is much more closely woven. Both are traditionally made of cotton.
here the cloth used for straing is called cheesecloth, a much looser weave than muslin
The definition of muslin is a loosely woven cotton. So if it has something else in it, it is a different type of fabric.