What is the ruling regarding prayers led by someone as an Imam while he was in a state of ritual impurity (janabah) due to his ignorance, and what are the implications of that for those who prayed behind him?

The prayer of someone who prays while in a state of major ritual impurity (junub) is invalid according to the majority of scholars, and they must make up that prayer. However, the prayer of the congregants behind an Imam who is in a state of major ritual impurity is valid, because the Imam's impurity is hidden from them and they cannot become aware of it. Any invalidating factor for the Imam's prayer that cannot be known does not invalidate the prayer of the congregants.