What is the ruling on the prayer of the congregants who sat during the last tashahhud and did not follow the imam into the fifth rak'ah after the imam remembered that he had not recited Al-Fatihah in one of the rak'ahs?

If the Imam stands for an extra Rak'ah by mistake, the congregants (Ma'mumin) should say "SubhanAllah" to him and not follow him, because following him would invalidate their prayer if they knew and did so intentionally. The prayer of those congregants who did not follow is valid, but repeating it is better to avoid scholarly disagreement.