Class QQmlApplicationEngine connects Qt.quit() signal to QCoreApplication::quit() and
QQmlApplicationEngine::exit() signal to QCoreApplication::exit(), but it does so with AutoConnection. This causes in some circumstances problems, which are described in Qt documentation.

Qt documentation on QCoreApplication::exit(),

"It's good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before "int main" calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop."