What is the ruling concerning my past prayers behind an Imam who substitutes the letter "dhāl" with "zāy" in Al-Fatiḥah, given that I only recently became aware of this?

The prayer of one who replaces the letter dhāl with zāy in Al-Fatiḥah is valid if he is unable to learn or cannot find a teacher. And it is permissible for someone similar to him to follow him in prayer. This is the opinion of most scholars. Some later Hanafi scholars went further, holding that it is permissible to pray behind someone who replaces dhāl with zāy or similar substitutions, because "it is a widespread affliction." It is permissible to follow this opinion, even if it is a less preferred one. Therefore, there is no need to repeat past prayers.