From the QNetworkReply docs:

QNetworkReply is a sequential-access QIODevice, which means that once data is read from the object, it no longer kept by the device. It is therefore the application's responsibility to keep this data if it needs to.
How can I save this data before it is read and lost?

I tried catching the QNetworkAccessManager::finished() signal but usually (not always) the data was already lost.