Must a missed rak'ah be completed after the imam prostrates for forgetfulness in the 'Isha prayer, which he prayed five rak'ahs?

If the Imam stands for an extra rak'ah, the congregants should alert him by saying "Subhan Allah" (Glory be to Allah) and should not follow him. If the Imam does not return, then whoever follows him intentionally and knowing that it is forbidden, their prayer is invalid. However, whoever follows him forgetfully or out of ignorance, their prayer is valid. This ruling also applies to the latecomer (masbuq): if he follows the Imam in the extra rak'ah out of ignorance or forgetfulness, his prayer is valid, but that rak'ah does not count for the rak'ah he missed before joining the Imam.