I'm by no means an expert, but I think that compared to Umayyad and Abbasid provinces like Egypt and the Levant, Persia experienced a much sharper contrast between it's native culture and that of the Arabs, and that relations between Zoroastrianism and Islam were worse than that between the other Abrahamic faiths and Islam. Unlike Jews and Christians, Zoroastrians eventually had their dhimmi status revoked under the Abbasids and were labeled kafirs. I believe, then, that religion was far less syncretic in Persia than elsewhere in the Caliphate, and that Muslim converts didn't retain many tenets of their prior faith.

"The Zoroastrian Diaspora: Religion and Migration," John Hinnells