I remember one where the person with Alzheimer's might have been a time traveler (this would be why I read it: it promised a spec fic element). iirc, the person with Alzheimer's/time traveler (it was that sort of trendy YA magical realism/surrealism book that nobody can actually write except Margaret Mahy, or in a completely different mood, Ellen Raskin, so it was never clear whether there was an ACTUAL spec fic element or if it was all bait-and-switch) was largely uninterested in/unaffected by the narrator, and the big reveal (the Terrible Truth that we learn by hanging out with People With Problems) was that the narrator's beloved dog had been responsible for the death of the narrator's beloved mother (she tripped over him and broke her neck), and that's why the narrator's hateful father got rid of the narrator's beloved dog. (The narrator had had traumatic amnesia and repressed the beloved mother's death, and so couldn't understand why his/her father hated the dog.) Everybody ended up exactly as dysfunctional as they were at the beginning, only with even more guilt and angst. And the person with Alzheimer's/time traveler disappeared.