I would aim for relaxed and calm, just in front of the vertical but soft and supple to the contact.
Changing shape implies an inconsistent contact or a variation in acceptance of the contact, that's why they comment on it. However, it can be caused by the horse misunderstanding the contact, being uncertain of what you want, or the rider's hands being unsteady in themselves.
Noosa's favourite evasion is to overbend behind the contact, but she will take it forward eventually
