After I got the latest code from git 5.4 branch, Qt will assert in QtFontFamily::ensurePopulated():

void QtFontFamily::ensurePopulated()
{
    if (populated)
        return;

    QGuiApplicationPrivate::platformIntegration()->fontDatabase()->populateFamily(name);
    Q_ASSERT_X(populated, Q_FUNC_INFO, qPrintable(name));
}

When the assertion occurred, The font name is "Simsun". Since when EnumFontFamiliesEx() is called with "Simsun", the font family name in its callback will be "宋体" which is alias to "Simsun". But that will prevent QtFontFamily.populated of "Simsun" to be set to true.