Cinderella's shoes shaped to her figure doesn't mean they won't fall off in any case.Otherwise we can't take off our shoes every night.
In the story,the fairy godmother's magic charm turned Cinderella's rags into a beautiful gown.But when 12 O'clock sounds,everything will turn back to the original.Under this emergency,Cinderella runs and loses one of her glass shoes.
It's normal for a pair of suitable shoes are off.