Update to iOS 15 Preventing Table Cells from Being Reused
## 🔙 Regression

<!--- Summary description of the regression --->

Prior to the iOS 15 release, I had a table that worked fine. Now I’m seeing sporadic cases where cells are populated on initial load (first image) but aren’t being reused after scrolling off and on the screen (second image). Adam Roth and Daniel Higgins are missing from the 2nd image. Your help is appreciated! Thanks.

### Old (and correct) behavior

![image](https://user-images.githubusercontent.com/12520398/143229888-7c73f73d-d0e5-4c6f-8f72-ac17e5dd7a89.png)

### Current behavior

![image](https://user-images.githubusercontent.com/12520398/143230133-97f20d4e-74b9-484c-b7eb-e099f9ed64d1.png)

(Adam Roth and Daniel Higgins are missing).

### Reproduction steps

Some pertinent code:

public override UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
{
var cell = tableView.DequeueReusableCell(reuseIdentifier, indexPath) as MvxTableViewCell;
if (ItemsSource is ObservableCollection<GroupedObservableCollection<string, T>> observableCollection
            && indexPath.Section < observableCollection.Count
            && indexPath.Row < observableCollection[indexPath.Section].Count
        )
        {
            try
            {
                cell.DataContext = observableCollection[indexPath.Section][indexPath.Row];
                cell.BackgroundColor = ColorConversion.ConvertFromMvxToUIColor(LightOrDarkTheme.BackGroundColor);  // 6529
            }
            catch (Exception)
            {
                return cell;
            }
        }

        return cell;
}

### Configuration

**Version:** MvvmCross 5.6.3

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