The final s of tās (the character of the second future) and as 'to be' is elided before an affix beginning with s ,
As कर्त्तासि, कर्त्तासे, त्वमसि, ष्यति से ॥ See 6.4.111. In से the अ and स् both of अस् have been elided, the अ by 6.4.111, and स् by the present sutra,so that the mere suffix से remains, which however here is a finite verb: hence the स, is not changed to ष ॥ See 8.3.111.,