Does praying aloud upon the Prophet (peace be upon him) during the recitation of Al-Fatiha and the surah thereafter invalidate the prayer, and is it obligatory to repeat it?

The Imam's act of interrupting the recitation during prayer to send blessings upon the Prophet, peace and blessings be upon him, is an innovated bid'ah (heresy) because it is not authentically reported from the Prophet, peace and blessings be upon him, who said: "Pray as you have seen me praying." The prayer is invalidated by this act because it is speech outside of the customary utterances of prayer. Since the Imam's prayer is invalid, your prayer behind him is also invalid and must be repeated.