The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “First: Sunnah prayers have flexible rulings.
Second, the mistake in recitation may fall in Surat Al-Fatihah or the recitation after it. If the mistake is in Al-Fatihah and it changes the meaning, then this should be repeated because reciting Al-Fatihah in a correct way is an obligation. But if mistake of the imam’s recitation did not change the meaning of Al-Fatihah then it is forgiven.
As for the mistake in recitation after Al-Fatihah, if the imam deliberately did this mistake that changes the meaning, then Prayer is invalid. But if he unintentionally did this mistake, then this Prayer is valid as recitation after Al-Fatihah is Sunnah.