الْفَتْحُ has fāʾ after the article, a moon letter, so the lām is pronounced clearly.
النَّاسَ has nūn, a sun letter: the article lām assimilates into a doubled nūn. Its ghunnah comes specifically from the doubled nūn, not from every sun-letter assimilation.
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Al-fatḥ and an-nās: compare the article
Al-Kāfirūn and An-Naṣr: Applied Madd, Idghām, Ghunnah and Waqf
الْفَتْحُ has fāʾ after the article, a moon letter, so the lām is pronounced clearly.
النَّاسَ has nūn, a sun letter: the article lām assimilates into a doubled nūn. Its ghunnah comes specifically from the doubled nūn, not from every sun-letter assimilation.