Is the action of the congregant who saw the Imam's 'awrah uncovered during prostration, then intended to pray alone and completed the prayer individually, and then informed the Imam after the prayer, correct?

The prayer of the ma'mum (follower) is valid because he separated from the imam (leader) due to the excuse of his awrah (private parts) being exposed. The permissibility of separating from the imam due to an excuse has been stipulated by jurists. As for the imam, his prayer becomes invalid if the exposure of the awrah is excessive and prolonged, and the worshipper does not cover it. If the imam's prayer becomes invalid, then the prayer of the ma'mumin who completed their prayer behind him does not become invalid if the matter was hidden from them. Thus, any nullifier of the imam's prayer, if it is hidden from the ma'mum, does not invalidate the ma'mum's prayer.