So, after that, Kairi lost her spellbook. But then, at the end of the game, I loaded that moment, tried again, and the book was there. Is this a bug?
I don't think it's a bug. You probably mixed up the starting setups (saves).
But yeah, I've never heard of that happening.
За то что нейтралы не бегают за единичкой, для меня это уже стоит того