Why was a "fatḥah" placed on the last word "al-mushrikīn" in the noble verse: "Lām yakuni allatheena kafaroo min ahli al-kitābi wa-al-mushrikīna munfakkeena," even though it is a sound masculine plural and in the genitive case?

"المشركين" (al-mushrikīn): it is in the genitive case, and its sign of jarr (genitive case) is the yaa' (ي). The fatḥa (fatha diacritic) on the noon (ن) of "al-mushrikīn" is not a sign of grammatical inflection, as the noon in the sound masculine plural is always open (i.e., has a fatḥa). Its inflection is by letters (harf), based on the saying of Ibn Malik: "And the noon of a plural and what is appended to it Open it and say that whoever pronounced it with a kasra has erred."