I don't think it's a common outcome in AS. I believe it's a possible outcome with any chronic inflammatory disease. What little I know about it derives mostly from a conversation I had with an oncologist friend. (Cachexia is very common in cancer patients.) It was shortly after my AS diagnosis, and she warned me, because I was at that time and had always been, a very thin person, to pay attention to my diet and exercise and not allow myself to lose any weight or muscle mass. She left me with the impression that there's this threshold that, once crossed, is almost impossible for a person suffering from chronic disease to recover from. Note that she needn't have worried about me. Middle age combined with the pandemic have worked together to keep me well above any such threshold.