Bad QTextDocument rendering when using QPixmap with Qt::transparent fill and QT_XCB_NATIVE_PAINTING set

Ever since I upgraded an application to Qt5 I have seen bad rendering on some user environments, where the fill with Qt::transparent combined with the QT_XCB_NATIVE_PAINTING set to 1. seems to behave badly and not do proper antialising.

Take this sample code which is supposed to just open a window with a Hello printed:

#include <QtCore/QString>
#include <QtGui/QColor>
#include <QtGui/QPainter>
#include <QtGui/QPixmap>
#include <QtGui/QTextDocument>
#include <QtWidgets/QApplication>
#include <QtWidgets/QLabel>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QWidget>


QPixmap getPixmap()
{
    QTextDocument text;
    text.setPlainText("Hello");

    QPixmap pm(100, 100);
    pm.fill(Qt::transparent);

    QPainter painter(&pm);
    text.drawContents(&painter, pm.rect());

    return pm;
}

int main(int argc, char** argv)
{
    QApplication app(argc, argv);
    QMainWindow mw;
    QLabel dummyLabel;
    QFont f = dummyLabel.font();
    f.setStyleStrategy(QFont::NoSubpixelAntialias);
    dummyLabel.setFont(f);
    dummyLabel.setPixmap(getPixmap());
    mw.setCentralWidget(&dummyLabel);
    mw.show();
    return app.exec();
}

Here's how it looks on a RHEL8 machine without QT_XCB_NATIVE_PAINTING set:

Here's how it looks on some RHEL8 machine with QT_XCB_NATIVE_PAINTING set to 1:

I did debug, but couldn't get to the conclusion of what is to blame. Some of what I investigated:

1. SSHing into a VM where an user reports the issue ("buggy" VM) does not reproduce it, only when running directly from the VM
2. SSHing into a non-buggy machine from the "buggy" VM does not reproduce the issue
3. This did not happen on Qt 4.6 (I know QT_XCB_NATIVE_PAINTING didn't exist back then, but then it was also the default)
4. Users that get this bug can reproduce it from their VMs in RHEL8/RHEL7/CentOS7 etc, seems to be tied to how the display is setup or something they might have installed