The forms يَفْعَلْنَ and تَفْعَلْنَ already end in the nun of feminine plural, so their written form remains unchanged here.
هُنَّ يَكْتُبْنَ ← لَمْ يَكْتُبْنَ
They (f.) write/will write → They did not write.
No visible change does not mean that لَمْ did nothing: the meaning has changed to past negation.
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The forms يَفْعَلْنَ and تَفْعَلْنَ already end in the nun of feminine plural, so their written form remains unchanged here.
هُنَّ يَكْتُبْنَ ← لَمْ يَكْتُبْنَ
They (f.) write/will write → They did not write.
No visible change does not mean that لَمْ did nothing: the meaning has changed to past negation.