QNetworkRequest::CacheLoadControlAttribute PreferNetwork still loads from cache even if resource was modified on server

We use Nginx server to download our files from. It supports all those cache control headers (etag, last-modified, if-modified-since, etc.)

We use QNetworkDiskCache.

For QNetworkRequest, we use this (just to make sure):

request.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferNetwork);

Steps to reproduce bug:

1. Download some resource. It will be cached.
2. Change resource on server.
3. Try to download it again. Bug: Qt does not even try to make any request to server to check if the resource is modified or not. It just uses cache as if PreferCache was specified.