da is substituted for the final s of a root with the exception of as 'to be' , before the personal ending ti (t) when it stands at the end of a word.,
The स् of a root can stand at the end of a word when the Personal-ending is dropped. Thus अचकाद् भवान्, अन्वशाद् भवान्, from the roots चकास् and शास् ॥ The Personal affix is elided by 6.1.68, and thereby स् comes to stand at the end of a Pada.
Why do we say 'before तिप्'? Observe चकास् formed by क्विप् affix added to the root.
Why do we say 'with the exception of अस्'? See आप एवेदं सलिलं सर्वम् आः ॥ Here आः is the Imperfect 3rd Person singular of अस् ॥ The ईट् is not added as a Vedic irregularity. See 7.3.93, 7.3.97.,