“Multiple personality disorder” probably doesn't exist
I think quite a few identity-centered mental illnesses might actually be particular delusions, and in some cases we may be grouping people subject to one mental abnormality with others who are under the delusion they have that abnormality. That could explain why certain subsets of people exhibit symptoms which don't match the normal presentation. For example, and anecdotally based on my personal experience, people under the delusion of being transgender get grouped together with people who are actually transgender. The "real" transgender people behave about the same way before and after their transition. They carry themselves like their gender, they talk like it, they react like it, and the biggest change after transitioning is them getting happier. The ones I hypothesize are actually subject to a delusion of being transgender don't act like that. Their personality shifts radically after transitioning and they behave like an exaggerated caricature of their target gender and have a very difficult time navigating small group dynamics of their target gender.I've seen similar patterns with people whose presentation of disorders like DID and Tourette's Syndrome differ radically from the clinical symptoms but match the pop culture presentation.