Good point.
However, all sankhara are anicca: “Sabbe sankhāra aniccā“.
Even though citta sankhara (same as mano sankhara) cannot lead to rebirth, they are still anicca. Even an Arahant would generate all three types of sankhara to live in this world until the death of the physical body. All sankhara are stopped only at Parinibbana.
– Such sankhara without kammic energy are called kiriya (just actions) too.