The source gives two different recovery rules:
• If the additional Surah was missed in the first two rak'ahs of a four-rak'ah Fard such as Isha, it is recited in the later rak'ahs together with al-Fatihah, while Sujud al-Sahw is still required for the missed Wajib placement.
• If al-Fatihah itself was missed in the first two, it is not repeated a second time in the later rak'ahs merely as compensation; Sujud al-Sahw is performed.
Do not treat the missed Surah and the missed al-Fatihah as the same recovery case.