When the milestones changes in middle of the gamr, that is not good. Hope you agree with me. In war and race you must follow rules.
During any commitment and communications you must Define this what required. If there is any deviation that should be communicated and corrected.
It doesn’t matter what technology and product you are working. Have a “change control board” to check if everything valid and acceptable, when requirement is added or deleted.
Continuous tracking of the product by the stakeholders and all other people will yield better results