Does the prayer become invalid if the worshiper is unable to complete the Abrahamic prayer in the last Tashahhud due to the imam prostrating for forgetfulness?

Scholars differed regarding the ruling on invoking blessings upon the Prophet (peace be upon him) in the final tashahhud. The Hanafis and Malikis held that it is a sunnah, and it is sufficient for the congregant that the Imam performs it. Whereas the Shafi'is and Hanbalis held that it is one of the pillars of the prayer, without which the prayer is not complete. The pillar part of it is: "Allahumma salli 'ala Muhammad" (O Allah, send blessings upon Muhammad), and what comes after it is sunnah. Accordingly, your prayer is valid according to the first opinion, and invalid according to the second if you did not perform the part considered a pillar. It is preferable to repeat it as a precaution. There is no harm in adopting the less preferred opinion after the action has occurred.