Is it correct that a Namaz is valid if a wajib is missed and Sajda sahu is forgotten unintentionally?

Once a mufti sahab of our area was doing Imamat for one of the farz Namaz and he missed a wajib in the namaz. but didn’t do Sajda sahu. We informed him that he had not done a sajda sahu even after missing the wajib. He said that he was aware of missing the wajib in the namaz and also that he had to do a Sajda sahu for that. But during the course of Namaz, he forgot about that. He said that sajda sahu is wajib if some one forget some wajib unintentionally and after that if he unintentionally forgets of doing Sajda sahu too, then that Namaz is fine and don’t have to repeat it. I confirmed this with some other Mufti Sahab, He said that it is not correct. Please confirm who is correct.