I have some experience with two people who had DIS/multiple personalities and with the (non-)acceptance of psychiatrists and psychologists. One person might get a borderline disorder instead of DIS. At least Frank Putnam's MPD book has large stretches that are not good or factual or good science (and do sometimes lack patient centering) Edit: Read Huldra's comment about him, they highlight important consequences of his ideas.

I have a couple of theories of, but mostly it doesn't matter so much. What matters is how we want to interact with bodies that we recognize. We do ascribe to them a sense of continuity and permanence which obviously is wrong and yet not seldom useful. We do ascribe to them certain patterns of acceptable or estimated behavior, too. A clearly neurodivergent friend I have gets cut much more slack if he isn't finding the right tone of cultural civility, which is the right thing to do. Other people would get more scold for failing social cues.

What matters in interacting with DIS is that you yourself are still allowed boundaries and it isn't a failure not to get not communicated things. Though it is nice to talk to people as they want to be talked to, even if that varies.