I can't believe that my guess of her dying of growing pains because she grew up too fast was right on the money.
I like how the story isn't using Santa Claus as the ultimate solution to everything and is giving focus on the differences between Sanda's forms, sometimes you don't need the jolly old man to help you, a good friend by your side who understands you can be just as powerful, in a way it went full circle, Fuyumura wanted Santa Claus to help her, but who stood by her side in the end was her friend Sanda.