.Net core HttpClient bug? SocketException: An existing connection was forcibly closed by the remote host

There is a bug for .NET Core 2.1 Preview mentioning this issue. That may be the cause. However, I also notice that your setting of TLS is incorrect. You're currently enabling it, but overwriting all other protocols that have been set. Instead of this:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

You should be using this:

ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
// ----------------------------------^

I think that's a side issue, but worth fixing all the same.

Update

The referenced GitHub issue above has a discussion which eventually links to the official announcement for .NET Core 2.1 SDK Preview 2. It has the following to say:

Sockets Performance and SocketsHttpHandler

We made major improvements to sockets in .NET Core 2.1. Sockets are the basis of both outgoing and incoming networking communication. The higher-level networking APIs in .NET Core 2.1, including HttpClient and Kestrel, are now based on .NET sockets. In earlier versions, these higher-level APIs were based on native networking implementations.

...

You can use one of the following mechanisms to configure a process to use the older HttpClientHandler:

From code, use the AppContext class:

AppContext.SetSwitch("System.Net.Http.UseSocketsHttpHandler", false);

The AppContext switch can also be set by config file.

The same can be achieved via the environment variable DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER. To opt out, set the value to either false or 0.