Is the prayer of one who completed a rak'ah behind the Imam invalidated if he then rejoined the congregation to complete the prayer after the Imam remembered a missing rak'ah?

If the Imam says the salām before completing his prayer and then returns to complete it, those who joined late must follow him. The rakʿah you prayed individually before the Imam returned is considered void, and you must repeat it after the Imam says the salām and performs the prostrations of forgetfulness (sujud al-sahw). Therefore, you must repeat the entire prayer because you only performed three valid rakʿahs.