reactive: ToTask doesn't unsubscribe properly

This issue occurs in a combination of an async method with Concat, Take(1) and with ToTask at the end.

Rx version: 4.1.2

Observable setup:

Func<Task> asyncMethod = async () =>
                         {
                            await Task.Delay(500);
                            Console.WriteLine("Done");
                         };

var observable = Observable.Interval(TimeSpan.FromMilliseconds(200))
                           .Do(i => Console.WriteLine(i))
                           .Select(_ => Observable.FromAsync(asyncMethod))
                           .Concat()
                           .Take(1)
                           .Finally(() => Console.WriteLine("Finished"));

Subscribe produces the expected result:

observable.Subscribe();

// Output:
0
1
2
Done
Finished

The task returned by ToTask completes correctly after 1st item when using await but the Interval keeps emitting values:

await observable.ToTask();

// Output:
0
1
2
Done
3
4
5
... and so on