...thinks you're a totally different person. He thinks he's seeing "you" for the first time when you come back downstairs for the second time.

TIC. Beats me...dogs are funny. Our lab mix randomly started exhibiting extreme fear of my BIL years ago, after seeming to love him for the years prior to that. He never did anything to warrant the change in behavior; he just showed up one day and she instantly ran away from him. We think it may have been scent-related (he'd had a close encounter with a skunk just prior to the first instance of the dog being fearful of him), but we never could conclusively pinpoint the issue. Several years passed by and she went back to loving him again. Go figure.