Confused elements property in listview, after add new items

Hello, I can not fix the following hard bug:
I wrote a render for a frame with background gradient

public class CustomFrame : Frame
{
public static readonly BindableProperty StartColorProperty =
BindableProperty.Create(nameof(StartColor), typeof(Color),
typeof(CustomFrame), Color.White);

    public Color StartColor
    {
        get => (Color)GetValue(StartColorProperty);
        set => SetValue(StartColorProperty, value);
    }

    public static readonly BindableProperty EndColorProperty =
        BindableProperty.Create(nameof(EndColor), typeof(Color),
                                typeof(CustomFrame), Color.White);

    public Color EndColor
    {
        get => (Color)GetValue(EndColorProperty);
        set => SetValue(EndColorProperty, value);
    }

    public CustomFrame(){}

}

[assembly: ExportRenderer(typeof(CustomFrame), typeof(CustomControls.CustomFrame.CustomFrameRenderer))]
namespace iOS.CustomControls.CustomFrame
{
public class CustomFrameRenderer : FrameRenderer
{
private CGColor startColor;
private CGColor endColor;

    CAGradientLayer gradientLayer = new CAGradientLayer()
    {
        StartPoint = new CGPoint(0, 0.5),
        EndPoint = new CGPoint(1, 0.5)
    };

    protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
    {
        base.OnElementChanged(e);

        if (e.NewElement == null) return;

        startColor = ((DPP.CustomControls.CustomFrame)Element).StartColor.ToCGColor();
        endColor = ((DPP.CustomControls.CustomFrame)Element).EndColor.ToCGColor();
    }

    public override void Draw(CGRect rect) //bug with update color
    {
        Element.ForceLayout();

        if (Element == null) return;

        gradientLayer.Frame = rect;
        gradientLayer.Colors = new CGColor[] { startColor, endColor};
        gradientLayer.MasksToBounds = false;

        NativeView.Layer.InsertSublayer(gradientLayer, 0);

        base.Draw(rect);
    }
}

}

In component binding
<c:CustomFrame BackgroundColor=" StartColor="{Binding StartColor}" EndColor="{Binding EndColor}">

on page set itemsource
Items = new ObservableCollection
{
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.Blue },
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.FromHex("#f51d2f")},
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.Blue },
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.FromHex("#f51d2f")},
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.Blue },
new FrameModel { StartColor = Color.FromHex("#db2e20"), EndColor = Color.FromHex("#f51d2f")},
};

listView.ItemsSource = Items;

private async void LoadItems()
{
if (isLoading) return;

        isLoading = true;

        await Task.Delay(4000);

        for (var i = 15; i > 0; i--)
        {
            Items.Add(new FrameModel { StartColor = Color.FromHex("#ff6423"), EndColor = Color.FromHex("#ffb02f") });
            lastItem++;
        }

        isLoading = false;
    }

after load new items, items confused colors, if scroll to start, elements behave as before (without mixing colors)

I will be very grateful for your ideas how you can solve this problem, my attempts to fix it dried out

sorry for bad english