The nasal of 1. daṃś 2. saṃj and 3. svañja is elided before the gaṇavikaraṇa śap of the roots of the bhvādigaṇaḥ (1st Conjugation).,
Thus दशति, सजति and परिष्वजते for the change of the स् of स्वञ्ज् to ष see 8.3.65,
दंश-सञ्ज-स्वञ्जाम् शपि
Adhyāya 6 · Pāda 4 · Rule 25