I fixed the problem I hid the subwindows before entering fullscreen mode and show them again after exiting fullscreen mode. A single resize event still occurs per subwindow, but the window is the same size as it was originally so it does not matter in my case. In the docs, under QWidget, QSize it states:

If the widget is visible when it is being resized, it receives a resize event (resizeEvent()) immediately. If the widget is not currently visible, it is guaranteed to receive an event before it is shown.