Huh! TIL. That said, I've heated a lot of soup - without using the "Soup" button, mind - and not experienced the problem you describe.

IANAMS (I Am Not a Microwave Scientist), but observationally it seems to me that bowl material has most to do with microwave results. I have a few bowls (and mugs) I never use in the microwave, because they seem always to get hotter than their contents. Later, I'll try one of them on "Soup" to see if it makes a difference.