The doubled mīm of فَأَمَّا has its own ghunnah; it is not a nūn-sākin rule.
In مَنْ ثَقُلَتْ, sākin nūn comes before thāʾ, which requires ikhfāʾ with ghunnah.
Identify the two causes:
فَأَمَّا — doubled mīm.
مَنْ ثَقُلَتْ — nūn sākin before thāʾ.
The same audible nasality can arise from different technical rules.
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Two sources of nasalization
Al-Qāriʿah and Az-Zalzalah — Comparative Tajwīd Practice
The doubled mīm of فَأَمَّا has its own ghunnah; it is not a nūn-sākin rule.
In مَنْ ثَقُلَتْ, sākin nūn comes before thāʾ, which requires ikhfāʾ with ghunnah.
Identify the two causes:
فَأَمَّا — doubled mīm.
مَنْ ثَقُلَتْ — nūn sākin before thāʾ.
The same audible nasality can arise from different technical rules.