Using Tibet binding on a iOS UILabel AttributedText property gives weird results in comparison to Fluent binding
## 🐛 Bug Report

As discussed in Discord with @Cheesebaron regarding re-triggering value converters in Fluent bindings, I've tried using a Tibet binding instead with AttributedText and got very strange results. Instead of getting my attributed string as returned in my value converter with the correct text, it gets returned with `{` postfixed.

Also spans are not working anymore as in my value converter, I possibly return an attributed string with the original text, or an attributed string with a `*` prefixed and a foreground color span on that character. The text that shows is then `*{` in all black instead of `* Behaald diploma` with the `*` in red for example.

With Fluent binding, I just get `* Behaald diploma` with the `*` in red as expected.

### Expected behavior
Tibet binding returns the correct attributed string.

### Reproduction steps

```
public class RequiredAttributedStringValueConverter : MvxValueConverter<string, NSMutableAttributedString>
{
    protected override NSMutableAttributedString Convert(string value, Type targetType, object parameter,
        CultureInfo culture)
    {
        var required = (bool?)parameter;

        var mutableString = new NSMutableAttributedString(value);
        if (required.HasValue && !required.Value)
        {
            return mutableString;
        }

        value = $"* {value}";
        mutableString = new NSMutableAttributedString(value);
        mutableString.AddAttribute(UIStringAttributeKey.ForegroundColor, Colors.Red.ToPlatformColor(),
                new NSRange(0, 1));

        return mutableString;
    }
}
```

Fluent binding:
`set.Bind(_lblEducationYear).For(v => v.AttributedText).To(model => model.LblEducationYear).WithConversion<RequiredAttributedStringValueConverter>(((OnboardingEducationInputListItem)DataContext).ShouldValidateFields)`

Tibet binding:
`set.Bind(_lblEducationName, "AttributedText RequiredAttributedString(LblEducationName, ShouldValidateFields)")`

### Configuration

**Version:** 8.0.2 (can't migrate yet since project is still not on .NET)

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