Y'know. I just realized something. Considering the Sephiroth you see throughout most of FF7 isn't actually Sephiroth. And the Sephiroth you see killing Aeris isn't actually Sephiroth. It was all Jeneova looking like Sephiroth, thinking with him and acting for him. So it probably stands to reason that the Sephiroth in AC was more Jenova anyway. So its not like he even "came back" in that movie