Tanwin resembles a still n-sound in connected pronunciation, but its stopping rule varies with the type of tanwin. With ordinary fathah tanwin, the final n-like sound disappears at a pause and is replaced by the sound of madd alif.
«عَلِيمًا» — its fathah tanwin is heard as an n-like ending when joined, while a stopped pronunciation takes the alif-length ending.
«نِسَاءً» — the same category calls for the specified stopped sound rather than merely removing the n-like tanwin without further consideration.
This is a rule about pronunciation rather than rewriting the Qur'anic spelling. The written form remains authoritative for the word, while its connected and stopped sounds differ. Words ending in ta marbuta need to be recognized as a separate case.