Well not necessarily. Just because it hasn't happened yet, doesn't mean it can't.
But Jmadeline posted a picture of her Shinku's neck
here, on this thread-- who doesn't have a soft torso, and from the pictures, her Shinku's neck HAS slightly melted especially compared to the other picture showing no melt.
Also, it definitely happens with pullips own soft-torso bodies, as my Anne has it and she is still on her original body. My opinion is that it can happen with any body, it just is most common with soft obitsu, but especially the new type of soft obitsu.