Fluent binding does not work in the nested view scenario
## 🐛 Bug Report

We have a complex root view layout consisting of a few nested views. Taking inspiration from the https://stackoverflow.com/questions/64156979/mvvmcross-view-inside-another-view-or-the-equivalent-to-a-caliburnmicro-conduc/64739282#64739282, we have setup a root view with embedded views with their `DataContext` bound to the corresponding viewmodels.

A simplified scenario in the modified Playground.Wpf app is as follows. **All sources below are in the https://github.com/ondrejsv/MvvmCross/commit/d03c4903138109603ea983e591891b51dfb1d558 for testing.**

The new `TestView` contains:
```
        <StackPanel>
            <Label x:Name="TitleLabel" />
            <Label>Static test of the TestView</Label>
        </StackPanel>
```

its very simple ViewModel:
```
    public class TestViewModel : MvxViewModel
    {
        public string MyTitle => "Binding Works!";
    }
```

The `RootView` has
```
<local:TestView DataContext="{Binding TestViewModel}" /> 
```

and its `RootViewModel` is amended with:
```
        private TestViewModel testViewModel = new TestViewModel();
        public TestViewModel TestViewModel
        {
            get
            {
                return testViewModel;
            }
        }
```

Now, when the **fluent binding is used** in the TestView, **it does not work**
```
        private void TestView_Loaded(object sender, RoutedEventArgs e)
        {
            using (var set = this.CreateBindingSet<TestView, TestViewModel>())
            {
                set.Bind(TitleLabel).For(v => v.Content).To(vm => vm.MyTitle);
            }
        }
```

We see that there is only the static text shown from the nested `TestView` but not the bound one:

![image](https://user-images.githubusercontent.com/747563/127288114-a82b663d-1e6d-4e4a-80d0-384449885f8b.png)

The WPF designer evens fails in both views:

![image](https://user-images.githubusercontent.com/747563/127287257-e06b9b53-cdd9-4ee2-8bc5-84b9da595c21.png)

**Should we switch to the XAML binding, it works as expected**:
```
            <Label x:Name="TitleLabel" Content="{Binding MyTitle}" />
```

![image](https://user-images.githubusercontent.com/747563/127288604-ff12ec5b-098b-45e4-ba37-8c11f50d7fdf.png)

We see the bound text "Binding Works!" from the viewmodel.

The WPF designed works again as well.

**The fluent binding works as expected when the view is not used as the nested.** E.g. when the TestViewModel becomes the root VM by `RegisterAppStart<TestViewModel>();`, the binding works (the "Binding Works" text is present):

![image](https://user-images.githubusercontent.com/747563/127286808-daab72db-91d2-487f-b91e-b606941fa846.png)


### Expected behavior

The fluent binding should work and the WPF designer should not fail.

### Reproduction steps

All repro steps with the sources codes are in the bug description.

### Configuration

**Version:** 8.0.2

**Platform:** 
- [ ] :iphone: iOS
- [ ] :robot: Android
- [X] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms
