Must one make up the 'Ishā prayer if the congregant (ma'mūm) said the salām with the Imām before completing the missed rak'ah out of ignorance, and if it is obligatory to make it up, when should that be?

An extra rak'ah due to forgetfulness does not invalidate the prayer, for in the hadith, the Prophet (peace be upon him) stood up for a fifth rak'ah out of forgetfulness and did not order his companions to repeat the prayer because they were unaware of the ruling. You are not required to repeat the prayer if you completed it immediately after the Imam's salutation and had to perform the prostration of forgetfulness. If you repeat it as a precaution, that is better. However, if a long time has passed, you must make it up immediately to clear your conscience. We advise you to seek religious knowledge to correct your worship.