Neither is Winnie the Pooh or The Muppets, Star Wars, Indiana Jones, Tron, or Spiderman.... so I'm not seeing the relevance.
And given that many of the "original" Disney Princesses are lifted from Fairytales or literature, they're not technically original, either: Ariel, Belle, Cinderella, Sleeping Beauty, Snow White, Mulan, Anna, Elsa, Tiana, Jasmine, Rapunzel... All are rooted in Fairytales, legends, or literature. Pocahontas was based on a real person.
So, if we want to be technical, the vast majority of the Disney Princess "original material" is more or less fanfiction.
The only Disney original princesses I can think of are Merida, Sofia, and Elena.
Originality is just an arbitrary line that is being drawn. If Disney owns it, it is Disney's IP now until they sell it.
So yeah, Anastasia is a Disney Princess. So is Leia.