Shapes in multisampling layers draw "bounding box" when strokeWidth is constant instead of shape

Trying to use the QtQuick.Shapes with layer multisampling, it turns out it does not work if the shape strokeWidth is constant.

The examples for this kind of smoothing all use an animation on the stroke width, so the issue is hidden.

To reproduce, try replacing the NumberAnimations on strokeWidth in the shapes example file "sampling.qml" with a constant strokeWidth (like, strokeWidth: 5). Observe that some kind of bounding area is drawn instead of the shape. It goes wrong in both cases either using multisampling or supersampling. See screenshot attached. 

Also, with the strokeWidth enabled, there is a flickering where the same "bounding area" is briefly visible during the animation.