Folding a rowareawithheader


one of my pages contains some rowareas with header. They should be foldable, but the icon right of the title in the header is only visible, when I set the property ‘foldable’ to ‘true’. Although the default is ‘true’, it isn’t visible without setting the property.
But much worse is the fact, that the icon disappears after folding the rowarea. And it doesn’t appear after unfolding it again.
So, the rowarea remains foldable by clicking the title, but no icon is visible. For a user, who doesn’t know that the rowarea is foldable, it might be difficult to recognize that clicking on the title folds the rowarea.

Is there any setting I missed or anything I have to implement in my adapter?

Thanks for your help!