QDockWidget: Empty space appears after using restoreState

There is a very strange behavior with QDockWidget in my application after updating from Qt 5.6.2 to Qt 5.9.6.

Reproduction:
1. Start the attached program (code is based on qt "dockwidgets" example)
2. Select View / Save state
3. Close the customers and the paragraph window (menu View) -> Screenshot1_ok.png, all ok
4. Select View / Restore state -> windows re-appear, all ok
5. Close the customers and the paragraph window (menu View) -> Screenshot2_failure.png, failure

With Qt 5.6.2, all is right ...