QFileDialog::DontUseNativeDialog related behavior mistake

1. For QFileDialog::setWindowIcon, it not work when no QFileDialog::DontUseNativeDialog option, which is the default options, for example: 

QFileDialog dialog;
dialog.setWindowIcon(a.style()->standardIcon(QStyle::SP_ComputerIcon));
dialog.exec();

2. For QFileDIalog::show, it always work as widget-based implementation no matter QFileDialog::DontUseNativeDialog is set or not, for example:

auto* dialog = new QFileDialog();
dialog->setOption(QFileDialog::DontUseNativeDialog, false);
dialog->show();

I test it in 5.15.2 and 6.5.2, they have same mistakes.