[REG 6.4 -> 6.5-beta3] Exec'd QMessageBoxes from QMenus return immediately on macOS

When spawning QMenu and using it to create a QMessageBox, when exec-ing said messagebox it returns immediately with return value 0 (NoButton). Sometimes (but not always) the native-looking messagebox is visible for a frame or two.

This is new to Qt 6.5. In Qt 6.4, the Qt generated messagebox correctly pops up and blocks until the user presses a button (or closes the dialog with other means).

I attached a minimal sample below that demonstrates the behavior.

I could not confirm other platforms, seeing the switch from Qt-generated to native messageboxes I would assume this to likely be exclusive to macOS.