Translation:The girl comes in and sits down on the sofa.
onto the sofa, rejected, even after having been "brain washed" to use onto, even when it sounded awkward. Yes, it may sound awkward here, but I would argue that "ra" emphasize the direction and justifies the use of onto rather than on.
I see you posted this 4 months ago. I just tried it again and "couch" is still being rejected. Reported!