My understanding is that there is no kamma vinnana in an arahant. kamma vinnana is defiled consciousness; an arahant has only vipaka vinnana.
Like you said, an arahant still engages in kusala sankhara, which does not bring kamma vinnana.
Sankhara that bring kamma vinnana are apunnabhi sankhara, punnabhi sankhara, anenjabhi sankhara, ie all abhisankhara. An arahant engages in none of those.
In the following post, Lal explained why an arahant does not generate any kamma vinnana.
Mōha/Avijjā and Vipāka Viññāṇa/Kamma Viññāṇa
Best,
Lang