QMainWindow tabPosition bug

When I use QDockWidget, I found a bug. The tabbar position is wrong. I set tab position to North, but when I move the large dock widget into the small dock widget, I got an wrong result.

here is my code

DockOptions options = dockOptions() | QMainWindow::GroupedDragging;
QMainWindow::setDockOptions(options);
QMainWindow::setTabPosition(Qt::AllDockWidgetAreas, QTabWidget::North);

Please look at pictures.

system: macOS 10.12.2