The /z/ is missing in Indian Sanskrit, but probably not in Mitanni Sanskrit (bi-ir-ya-ma-aš-da/Priyamazdha). So, this is not the most critical. Some insular Indo-Aryan tribes did not migrate to South Asia, but into West Asia.
In Indian Sanskrit, the word jānu(knee) and jīvati(to live) have a same j(d͡ʑ). However, they are of different PIE sources. Avestan distinguishes this diffrenece. For example, zanu and juuaiti. So Avestan is not derived from Vedic, but may still be intricately related to other insular Indo-Aryan languages.