What is the ruling on one who stands to make up a missed prayer, and then it is found that the imam’s prayer was deficient?

The Shaykh : A man missed the 'Asr prayer. When the imam said the taslim and left, the man stood up to make up the prayer. The imam was informed, so he stood up and began to complete the prayer. The man joined him, considering that the prayer had not been broken off in terms of the ruling, given that the interval between the imam’s departure and the man joining him was short. If a masbuq (one who joins the prayer late) stands up to make up what he missed, and then it is found that the imam has a deficiency in his prayer which he completes, the ma'mum (one praying behind the imam) has a choice: if he wishes, he may join the imam, and if he wishes, he may continue making up what he missed. This is because he left for a valid legal excuse, so he has a choice between returning and proceeding. This is Fu'ad al-Juhani.