Changes the window flags of a Qt::Popup window are not reflected

When creating a window with Qt::Popup flags then changing it to a different set of flags still including Qt::Popup, the window does not reflect the change in flags visually despite being able to see by checking the flags that it has different flags.

This issue does not happen with other flag presets such as Qt::Dialog or Qt::Window, and only occurs when going from one Qt::Popup to another.