If you look at ch 4 again, she was wearing a winter uniform at the start, and so were the guys, but now in this uniform they aren't.
I guess that means that the demon was able to undo him dying after all, and for whatever reason she decided to put Kiyose in her body months after they started dating, or this is a 3rd parallel universe where she was their childhood friend AND she's dating Nogata, but wouldn't her body have a problem again?
Unfortunately there's not more detail so only the author knows for sure, maybe he wrote a note somewhere explaining it