What is the ruling regarding someone who prayed with a flawed ablution or whose ablution was nullified during the prayer, yet was unaware of it until after his death?

The doctrine of the majority of scholars, which is the authoritative legal opinion, is that the conditions for prayer do not drop due to forgetfulness or ignorance. So, whoever prays without ablution out of ignorance, their prayer remains a debt upon them, and they are obligated to repeat it during their lifetime. As for after death, if you were negligent in learning, then your matter is with Allah. But if you were not negligent, we hope that no sin will befall you.