What is the ruling on praying behind an Imam who softens the letter "ḍād" in the word "al-maghḍūb" from Al-Fatihah, making it close to a "dāl", knowing that I am proficient in reciting Al-Fatihah?

If the لحن (lisp/defect) in the letter ض (Dhaad) is merely an imagination, then the prayer is valid. The prayer of one who pronounces the ض (Dhaad) as a softened sound, close to the د (Daal), is also valid. However, if someone clearly substitutes the ض (Dhaad) with a د (Daal), then his prayer is invalid, as is the prayer of anyone praying behind him. Scholars have warned against obsessive doubt (waswasa) regarding the articulation points of letters, and have cautioned that exaggerating the pronunciation of a letter distracts from understanding the recitation.