What is the ruling on the Imam saying, "My Lord, forgive me and my parents," after reciting Al-Fatihah and before saying "Ameen" in prayer, and does prayer behind him become invalid, and is one who remains silent about this act sinful?

Supplicating while standing is disapproved and contrary to the Sunnah (definition). Standing is for recitation, bowing for glorification, and prostrating for supplication. This is based on the saying of the Prophet, peace and blessings be upon him: "Indeed, I have been forbidden to recite the Qur'an while bowing or prostrating. As for bowing, glorify the Lord, the Mighty and Majestic, therein. And as for prostrating, strive hard in supplication, for it is most likely that your prayers will be answered." The prayer itself is valid, but imams and worshippers should be alerted to the fact that this action contravenes the Sunnah, without neglecting congregational prayer.