finally after all this confusion, the reason why Cinderella's glass slippers didn't disappear at midnight was because they were a gift from the fairy godmother, they weren't a new form of something else unlike the carriage that she changed from a pumpkin. after midnight everything the fairy godmother use transmutation magic would change back, but since the shoes were simply made out of thin air not from an original item they wouldn't changed back when the clock hit midnight.