Red cross on output image in UWP app

[Sarnapa]

To be brief, I'm trying to implement some motion detection algorithm and in my case I'm working on UWP using portable version of AForge library to processing image. Avoiding this issue, I have problem with conversion SoftwareBitmap object (that I get from MediaFrameReader) to Bitmap object (and vice versa) that I use in my code associated with motion detection. In consequence of this conversion, I get proper image with big red cross in the foreground. Code below:

private async void FrameArrived(MediaFrameReader sender, MediaFrameArrivedEventArgs args)
{
    var frame = sender.TryAcquireLatestFrame();
    if (frame != null && !_detectingMotion)
    {
        SoftwareBitmap aForgeInputBitmap = null;
        var inputBitmap = frame.VideoMediaFrame?.SoftwareBitmap;
         if (inputBitmap != null)
         {
             _detectingMotion = true;
             //The XAML Image control can only display images in BRGA8 format with premultiplied or no alpha
             if (inputBitmap.BitmapPixelFormat == BitmapPixelFormat.Bgra8
                 && inputBitmap.BitmapAlphaMode == BitmapAlphaMode.Premultiplied)
             {
                 aForgeInputBitmap = SoftwareBitmap.Copy(inputBitmap);
             }
             else
             {
                 aForgeInputBitmap = SoftwareBitmap.Convert(inputBitmap, BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore);
             }
             await _aForgeHelper.MoveBackgrounds(aForgeInputBitmap);
             SoftwareBitmap aForgeOutputBitmap = await _aForgeHelper.DetectMotion();
             _frameRenderer.PresentSoftwareBitmap(aForgeOutputBitmap);
             _detectingMotion = false;
         }
    }
}

class AForgeHelper
{
    private Bitmap _background;
    private Bitmap _currentFrameBitmap;

    public async Task MoveBackgrounds(SoftwareBitmap currentFrame)
    {
        if (_background == null)
        {
            _background = TransformToGrayscale(await ConvertSoftwareBitmapToBitmap(currentFrame));
        }
        else
        {
            // modifying _background in compliance with algorithm - in this case irrelevant
        }
    }

    public async Task<SoftwareBitmap> DetectMotion()
    {
        // to check only this conversion
        return await ConvertBitmapToSoftwareBitmap(_background);
    }

    private static async Task<Bitmap> ConvertSoftwareBitmapToBitmap(SoftwareBitmap input)
    {
        Bitmap output = null;
        await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
        {
            WriteableBitmap tmpBitmap = new WriteableBitmap(input.PixelWidth, input.PixelHeight);
            input.CopyToBuffer(tmpBitmap.PixelBuffer);
            output = (Bitmap)tmpBitmap;
        });
        return output;
    }

    private static async Task<SoftwareBitmap> ConvertBitmapToSoftwareBitmap(Bitmap input)
    {
        SoftwareBitmap output = null;
        await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
        {
            WriteableBitmap tmpBitmap = (WriteableBitmap)input;
            output = new SoftwareBitmap(BitmapPixelFormat.Bgra8, tmpBitmap.PixelWidth, tmpBitmap.PixelHeight,
                                        BitmapAlphaMode.Premultiplied);
            output.CopyFromBuffer(tmpBitmap.PixelBuffer);
        });
        return output;
    }

    private static Bitmap TransformToGrayscale(Bitmap input)
    {
        Grayscale grayscaleFilter = new Grayscale(0.2125, 0.7154, 0.0721);
        Bitmap output = grayscaleFilter.Apply(input);
        return output;
    }
}

Certainly, I've tried to detect some errors using try-catch clauses. I've found nothing. But it seems to me that this problem is caused by casting WriteableBitmap to Bitmap and at this point I want ask if during this procedure it is applied special watermark in the shape of red cross. Thanks in advance for help.