What is the ruling on the prayer of a congregant who completed his prayer after his Imam forgot a prostration in the third unit of the 'Isha prayer, and then the Imam repeated the prayer?

Your prayer is valid because you caught a valid rak'ah with the Imam and separated from him at a point where it is permissible for you to do so; therefore, there is nothing upon you. It is permissible for a follower (ma'mum) to separate from the Imam based on a legitimate Shar'i evidence, and thus he is excused in this separation. If the Imam returns to complete his prayer, the follower has a choice: he may continue his prayer if he wishes, or he may return with the Imam. If the Imam gives the taslim before completing his prayer, and the followers rise to make up what they missed, then the Imam remembers that he has a deficiency and rises to complete it, the followers are given the choice between continuing to make up what they missed and returning with the Imam.