> FFM isn’t entirely muscle, but what other weight would be shed when losing FFM other than muscle?

I'm not an expert, but I have to imagine that most of it is muscle.

After dramatic weight loss, a person will probably lose some bone - particularly in the lower body - due to decreased loading.

I know body builders sometimes eat extremely high protein diets (more than 1 g/lbs of body weight) and lift quite hard to try to hang on to as much muscle mass as possible. And they still lose some when cutting.