QTextEdit::extraSelections do not work well on Linux/X11 when QTextEdit has explicitly set small font size

#include <QApplication>
#include <QTextEdit>

void highlight(QTextEdit *textEdit)
{
    QList<QTextEdit::ExtraSelection> highlights;
    QString word = "abcd";
    QString text = textEdit->toPlainText();
    int pos = text.indexOf(word);

    while (pos != -1)
    {
        int endPos = pos + word.length();

        auto highlight = QTextEdit::ExtraSelection();
        highlight.cursor = QTextCursor(textEdit->document());
        highlight.cursor.setPosition(pos);
        highlight.cursor.setPosition(endPos, QTextCursor::KeepAnchor);
        highlight.format.setForeground(QApplication::palette().color(QPalette::Link));
        highlights.append(highlight);

        pos = text.indexOf(word, endPos);
    }

    textEdit->setExtraSelections(highlights);
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QTextEdit w;

    // THE NEXT LINE!!!
    // If point size 12 or smaller is used, then every second
    // row in the text edit is not highlighted.
    // If commented out, then everything works fine.
    // If point size 13 or greater is used, then everything works fine.
    QFont f;
    f.setPointSize(12);
    w.setFont(f);

    w.setPlainText("abcd abcd abcd abcd abcd abcd abcd abcd "
                   "abcd abcd abcd abcd abcd abcd abcd abcd");
    highlight(&w);
    w.show();
    return a.exec();
}