The mask salesman makes off with Miyo's human face.
As far as the humans are concerned, Miyo has run away.
She sees how concerned everyone is, an regrets deciding to become a cat.
Especially when she realizes she's losing her ability to understand language.
Three days later, Miyo reappears.
Except it's not her. It's someone else wearing her face.
It's Kaoru's cat, Kinako. She was reaching the end of her lifespan anyway, and this lets her cheer up her owner, and live on as her daughter.
Win/win, right?