Why did the word "Thamud" appear in the nominative case after the preposition "fī" in Surat Adh-Dhariyat, while the word "ʿĀd" appeared in the genitive case after the same preposition in "wa-fī ʿĀd"?

"Thamud" in Surat Adh-Dhariyat has not been read with a damma (nominative case) in any of the ten Qira'at (readings); rather, it has a fatha (accusative case) because it is diptote (prevented from tanween). As for "Ad," it came in the genitive case because it is not diptote. A noun that is diptote does not take tanween, and it is put in the genitive case with a fatha instead of a kasra, unless it is in a genitive construction (mudaf) or defined with "al."