Bug in Observable.FromAsync

## Bug

### Which library version?

System.Reactive 5.0.0

### What are the platform(s), environment(s) and related component version(s)?

Windows, WPF, net5.0-windows10.0.19041

### What is the use case or problem?

```c#
async Task DoSomethingOnBackgroundThreadAsync()
{
await Task.Run(() => Thread.Sleep(1000));
}

//The following line is executed on the UI-thread, but unexpectedly the Handler is executed on
//a background thread, which led to hard to track down race conditions in our application.

Observable.FromAsync(() => DoSomethingOnBackgroundThreadAsync()).Subscribe(x => Handler());
```

A workaround for the problem is to pass DispatcherScheduler.Current to FromAsync, but it strongly feels like a bug, that should be fixed.