QDockWidget::restoreGeometry not working correctly when QMainWindow is maximized

I have a number of QDockWidgets, all docked in a single QMainWindow.

I have overridden the showEvent, and after passing the event on to the base class I am restoring the dock widget's geometry

void DockWidget::showEvent(QShowEvent* ev) 
{
    QDockWidget::showEvent(ev);

    QByteArray byte_array = settings_.value(id + ".geometry").toByteArray();

    LOG("rest: %s", QString(byte_array.toHex()));

    QDockWidget::restoreGeometry(byte_array);
}

In my QMainWindow::closeEvent I am calling saveSettings for each of my dock widgets

void MainWindow::closeEvent(QCloseEvent* ev) 
{
    QList<DockWidget*> dock_widgets = findChildren<DockWidget*>();

    for (DockWidget* dock_widget : dock_widgets)
        dock_widget->saveSettings();

    QMainWindow::closeEvent(ev);
}

In my dock widget's saveSettings function I write the result of saveGeometry to disk:

void DockWidget::saveSettings()
{
    QByteArray byte_array = QDockWidget::saveGeometry();

    LOG("save: %s", QString(byte_array.toHex()));

    settings_.setValue(id + ".geometry", byte_array);
    settings_.sync();
}

Whilst this does work when my QMainWindow is not maximized, when it is maximized the widgets aren't restored correctly.

In this image I have arranged my widgets just prior to closing. (linked because inline the image will be too large)

In this image I reload my app and the widgets' geometry is incorrectly loaded.

You can see in my functions above I log the geometry string being saved and loaded.

I have proven to myself that the settings are correctly saved and restored again, but somehow it's not working correctly

close the app; save the state:

save: 01d9d0cb000200000000053f000000640000077f000001a00000053f000000640000077f000001a000000000000000000780

open the app; restore the state: (the hex data here matches the saved one exactly)

rest: 01d9d0cb000200000000053f000000640000077f000001a00000053f000000640000077f000001a000000000000000000780

close the app again, having touched nothing: (the hex data is different now, as the geometry is different, see marks below)

save: 01d9d0cb000200000000053f000000640000077f0000014e0000053f000000640000077f0000014e00000000000000000780
                                                    ^                               ^

This doesn't happen when the window is not maximized.

Is this a bug in Qt, or am I not using the functionality correctly?

I am using Qt 5.5 on Ubuntu 16.04.