Ok, Matpat-level theorizing time.
If the slipper was a perfect fit, it’s safe to say that the slipper can come off. Even if your shoes are a perfect fit, enough force will kick them off, unless it’s tied down. Cinderella doesn’t have the luxury of tying her glass slippers on, so it’s plausible that, in her hurried escape, she shook her foot with enough force make the slipper fall off.
And that’s just a theory.