QMessageBox not modal when using Weston compositor

When using Qt Wayland (using QPA `-platform wayland`) and displaying a QMessageBox with parent set to a QWidget, the Dialog it is not shown as a modal Dialog (it does not stays on top) when using Weston. This is also reproducible when running Weston on my Desktop.

When using Gnome Mutter as Wayland compositor the parent Dialog becomes active, but the QDialog stays on top and no interaction is possible on the parent dialog (as expected).