In مَا تَعْبُدُونَ the following word begins with tāʾ, not hamzah; thus the madd in «مَا» is natural madd.
Contrast مَا أَعْبُدُ, where the next word begins with hamzah: connecting them produces madd munfaṣil.
Compare the last letter of the first word and the first of the next in each phrase; the shared shape «مَا» by itself does not determine the rule.
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Not every long vowel is munfaṣil
Al-Kāfirūn and An-Naṣr: Applied Madd, Idghām, Ghunnah and Waqf
In مَا تَعْبُدُونَ the following word begins with tāʾ, not hamzah; thus the madd in «مَا» is natural madd.
Contrast مَا أَعْبُدُ, where the next word begins with hamzah: connecting them produces madd munfaṣil.
Compare the last letter of the first word and the first of the next in each phrase; the shared shape «مَا» by itself does not determine the rule.