If an imam goes down into ruku while I’m finishing Fatiha and he comes up right as I am going down or before I go down does this invalidate my salah and I have to repray or does it just invalidate my rakat and I can add another rakat after the imam finishes?

Becareful of this, follow the Imam even if you didn't finish reciting because you have alot and you will miss the pillar, some of the people of ilm said that the rak'ah is nullified and u add a rak'ah and some said the whole salah is invalid and Allah knows best.