The ṣ substitution does not take place in the s of soḍha form of the root saha,
As परिसोढः, परिसोढुम्, परिसोढव्यम् ॥ This is an exception to 8.3.70.
Why do we say 'when सह assumes the form of सोढ' ॥ Observe परिषहते, विषहते ॥,
सनोतेः स्य-सनोः
Adhyāya 8 · Pāda 3 · Rule 115