QModbusTCP doesn't change state nor fire error when disconnected

[Mark81]

I'm facing a lot of issues using QModbus classes. For example, if I successfully connect to a device:

QModbusClient *_modbus;

bool ModbusClient::open(QString host, int port)
{
    if (_modbus)
    {
        _modbus->disconnectDevice();
        delete _modbus;
    }

    _modbus = new QModbusTcpClient(this);
    _modbus->setConnectionParameter(QModbusDevice::NetworkPortParameter, port);
    _modbus->setConnectionParameter(QModbusDevice::NetworkAddressParameter, host);

    _modbus->setTimeout(250);
    _modbus->setNumberOfRetries(1);

    connect(_modbus, &QModbusDevice::stateChanged,this, &ModbusClient::stateChanged);
    connect(_modbus, &QModbusDevice::errorOccurred,this, &ModbusClient::errorOccurred);

    return _modbus->connectDevice();
}

void ModbusClient::stateChanged(QModbusDevice::State state)
{
    qDebug() << "state" << state;
}

void ModbusClient::errorOccurred(QModbusDevice::Error error)
{
    qDebug() << "error"<< error;
}

then I disconnect the cable from the device, but neither the state changes to UnconnectedState nor the error signal is emitted.

So, how to detect such a disconnection?