Why is the letter "fa" vowelled with a "fatha" in the word "al-mustafeen" in the Quranic verse: "And indeed they are with Us, of the chosen (al-mustafeen), the best," even though the letter preceding the "ya" in a sound masculine plural is usually vowelled with a "kasra"?

The word "al-Mustafīn" (المصطفين) does not violate the rule. Its origin is "al-Mustafayīn" (المصطفَيِين), where the kasra-bearing ya (ي) was changed to an alif (ا) because it was vocalized and preceded by a fatha. Then, the alif was deleted due to the meeting of two silent letters. Ibn Malik referred to this rule by saying: "From a waw or ya, if vocalized, replace it with an alif after an immediately preceding fatha, if the following letter is vocalized."