Since being in the House of Black and White, Arya seems to be a little different. Sure, she traveled across the whole country and went to hell and back, but somehow after her battle in Braavos Arya had changed a lot of her behaviour:
- She switched her hair from traditional Arya to "The Waif"-hairstyle.
- She used to be right-handed, now she is fighting with her left hand (compare the Arya-Brienne fight in season 7, episode 3)
- she has few to no characteristics left in her
- she switched her communication style (from lying a lot to being honest)
One could assume that The Waif was victorious over Arya and is now impersonating Arya for a greater purpose.
Or did I miss the scene where The Waif finally gets killed? As far as I know the exact fighting scene is just the escape, followed by a "blackout fight" in the dorms where Arya sleeps.