QStackedWidget::removeWidget does not revert ovnership to application as stated in documentation:
" Note: Ownership of widget reverts to the application."

Code like this

QStackedWidget *stack = new QStackedWidget;
QFrame *frame = new QFrame(this);

qDebug() << frame->parent() << frame->parentWidget();
stack->addWidget(frame);
qDebug() << frame->parent() << frame->parentWidget();
stack->removeWidget(frame);
qDebug() << frame->parent() << frame->parentWidget();

produces output like this:

MainWindow(0xcdfb70) MainWindow(0xcdfb70)
QStackedWidget(0xfd6100) QStackedWidget(0xfd6100)
QStackedWidget(0xfd6100) QStackedWidget(0xfd6100)