Seen on Windows 7, Qt 5.4.1, 32-bit
Steps to repro:

1. Create 2 QMainWindow's on the same display
2. QWidget::move() them both to the top-left position on both monitors of a 2 monitor system
3. QWidget::showFullScreen() both QMainWindows into fullscreen mode
4. The QMainWindow on the secondary display will jump back to the primary display
5. QWidget::showNormal() both QMainWindows back to windowed-mode
6. The QMainWindows will be back to their normal size at the top-left position of each monitor
7. QWidget::showFullScreen() both QMainWindows into fullscreen mode again
6. This time the QMainWindow on the secondary display will stay on the secondary display when in fullscreen mode.

Not sure what the secondary QMainWindow is supposed to do - I'm QWidget::move()'ing it rather than QWindow::setScreen()'ing it - but the fact that it does something different the second time through seems wrong.