What is the ruling if the follower, upon rising to make up missed prayers, is still in the process of becoming fully upright when the Imam performs Sujud As-Sahw?

Primary text

If the follower has not fully straightened up (i.e., not completed istitimam) when the Imam performs the prostration of forgetfulness after the final Taslim, the follower must return (to sitting), provided the follower has not yet performed an action related to the missed parts.