The affix ṅīṣ is always used in the chandas (Vedas) in forming the feminine after the word bhu,
Thus विभ्वी, प्रभ्वी, सुभ्वी, शम्भ्वी ॥
But why not so in the case of स्वयभूः? Because it ends in long ऊ, while विभ्वी &c. are feminine of विभु &c. ending in short उ ॥ In fact the word उतः \after a word ending in short उ\ of Sutra 4.1.44 governs this also. The word भुवः is the ablative case of भु irregularly formed by the substitution of उवङ्; this form being confined to sutras only.,