PR dotnet/corefx#38918 removed _connection.ExtendWindow(bytesRead); from the response stream's ReadAsync:
https://github.com/dotnet/corefx/blob/c14b80faff694bae4e085bad221e2e83410e5f33/src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Stream.cs#L649-L652
but didn't remove it from Read:
https://github.com/dotnet/corefx/blob/c14b80faff694bae4e085bad221e2e83410e5f33/src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Stream.cs#L624-L628
I assume that's a bug that needs to be fixed in Read by removing the corresponding line?

cc: @geoffkizer