If x509 certificate has extension "Subject Alternative Name" we can read it but it not has a correct value. Code below should print type "QVariantMap" and method "toMap()" should be successful. Defect appears only on Apple devices, on other systems (Windows, Linux, Android) behaviour is correct. 

QList <QSslCertificate> myCerts = QSslCertificate::fromPath(" path to .der", QSsl::Der);

    if (!myCerts.empty())

        for(auto it : myCerts[0].extensions())

            if (it.name() == QLatin1String("subjectAltName"))

                qDebug() << "extension " << it.oid() << " have " << it.value().toMap().count() << "elemnts and type " << it.value().typeName();

On Windows:
extension "2.5.29.17" have 2 elemnts and type QVariantMap

On MacOS:
extension "2.5.29.17" have 0 elemnts and type QByteArray

Qt Plugin "Qt OPC UA" uses method "toMap()", so I'm assuming that the Windows behaviour is correct.