Agreed, even for the Sotapanna the akusala mula PS will operate at cuti moment.
Do you say that new kamma bhava is accumulated while doing kusala kamma? And this (kusala) kamma bhava is grasped via akusala mula PS at the cuti moment?
We should remind us that kusala kamma is neither dark nor bright with neither-dark-nor-bright results, it is kamma that leads to the destruction of defilements. Thus kusala kamma will not add to kamma bhava. That is the reason it is not mentioned in kusala mula PS.