A simple reproducer for testing:

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

    QTextDocument* doc = new QTextDocument;
    QString strWithBgd("<i><font style=\"font-size:72px\"><font style=\"background:#FFFF00\">イ雨ｴ</font></font></i>");
    QString strWithoutBgd("<i><font style=\"font-size:72px\">イ雨ｴ</font></i>");
    doc->setHtml(strWithBgd + "<br>" + strWithoutBgd);

    QTextEdit* text = new QTextEdit;
    text->setDocument(doc);
    text->show();

    return a.exec();
}

2 HTML strings are being tested: one with background and one without. And the result is what attached screenshot shows, that somehow background is rendered on top of the character on left side so that the character on left side is "clipped". And the string without background is rendered as expected.

It is reported that everything works fine back in 5.1. Customer provides a screenshot for comparison between 5.1 and 5.15 (I don't have 5.1 environment, so I haven't tested).

It seems like that under extreme condition, i.e. large italic character gets rendered into rendering area of other character, the z-ordering is somehow broken.