My current theory is that a bunch of the people that know about his amnesia are passing him around between "loops" to act as something like, "the only person he knows." Itsuki was maybe the one just before the current girl, so Shiota still has lingering memories of him. (/ her?) Itsuki's got to hold up their end of the deal, though, so they're not allowed to associate with him, hence pretending to not know him and blocking his number with a sort of pained look while also blushing at some of his comments.