What is the ruling on praying behind an Imam who pronounces the letter Raa lightly, close to a Waw, in Al-Fatihah, and am I required to repeat the prayer?

If you are not certain that the Imam changed the letter “Rā’” to “Wāw,” then do not pay attention to this matter, and your prayer is valid.

However, if it is confirmed that the Imam changes “Rā’” to “Wāw” in Al-Fatihah, this is a grammatical error (laḥn) that alters the meaning, and prayer behind him is not valid except for someone who makes the same mistake. So, whoever omits a letter from Al-Fatihah, or substitutes it, or makes a grammatical error that changes the meaning, is like an illiterate person, and it is not permissible for a reciter to pray behind him. Nevertheless, some scholars have held that the prayer of one who makes a grammatical error unintentionally is valid, and that it is permissible to pray behind him.