QMessageBox before app.exec leads to incorrect state

I show a message box before starting the main event loop. After closing the message box, the main application window appears in a bad state:

 - the window does not have focus

 - none of the widgets have focus, and clicking any of the line edits does not make them highlight as expected, yet entering text works.

The application gets to a normal state if I click into a line edit, then switch to a different application and back.

I attached a small example application reproducing the issue and screenshots of the behaviour:

 - 1. Startup: no window foxus, no line edit focus

 - 2. After clicking on the window and into the line edits: window has focus, line edits don't

 - 3. After switching to a different application and back: now also the focused line edit has focus highlight

The issue can be reproduced to some degree in Qt (without PySide) also. There the line edits behave correctly, but the application also starts without focus.