The niṣṭhā ta is changed to na after añc but not when it is in connection with an Ablative -5th case.,
Thus समक्नौ शकुनेः पादौ = सङ्गतौ; तस्मात् पशवो न्यक्नाः ॥
Why do we say 'when not in construction with an Ablative case'? Observe उदत्तमुदकं कूपात् = उद्धृतं 'drawn out'.
The word व्यक्तम् is from the root अञ्जू व्यक्तिम्रक्षण कान्तिगतिषु, and not from the root अञ्च्; and hence the Nishtha is not changed.,