C# - Unexpected pixel values from Graphics.DrawImage() result when dealing with alpha

Question

I need to perform a Graphics.DrawImage from a source to a target image, ideally from a smaller tile to a larger poster, but it doesn't matter in the specific scenario.

I write into an empty target image, and I set the g.CompositingMode = CompositingMode.SourceCopy; however, after the writing process, I get wrong values if the source image has an alpha channel.

I already checked for similar question and answers, but none could solve this situation.

There is not a single alpha value for all the source image; the expected result is an exact copy of the source image at the specified location of the target one.

The sample I assembled in order to test this is the following:

// Create a new bitmap.
const int bitmapSize = 5;
Bitmap 
    bmp = new Bitmap(bitmapSize, bitmapSize, PixelFormat.Format32bppArgb), 
    bmpDestDrawImage = new Bitmap(bitmapSize, bitmapSize, PixelFormat.Format32bppArgb);

#region create source image

// Lock the bitmap's bits.  
Rectangle rect = new Rectangle(0, 0, bitmapSize, bitmapSize);
System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.WriteOnly, bmp.PixelFormat);

// Get the address of the first line.
IntPtr ptr = bmpData.Scan0;

// Declare an array to hold the bytes of the bitmap.
int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
byte[] rgbValues = new byte[bytes];

Random rnd= new Random(0);

for (int counter = 0; counter < rgbValues.Length; /**/)
{
    rgbValues[counter++] = (byte)rnd.Next(0,255);
    rgbValues[counter++] = (byte)rnd.Next(0, 255);
    rgbValues[counter++] = (byte)rnd.Next(0, 255);
    rgbValues[counter++] = (byte)rnd.Next(0, 255);
}

// Copy the RGB values back to the bitmap
System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, bytes);

// Unlock the bits.
bmp.UnlockBits(bmpData);

#endregion

using (Graphics g = Graphics.FromImage(bmpDestDrawImage))
{
    // From the Graphics.CompositingMode Property documentation page
    g.CompositingMode = CompositingMode.SourceCopy;
    g.TextRenderingHint = TextRenderingHint.SingleBitPerPixel;
    
    g.DrawImage(bmp, new Rectangle(0, 0, bitmapSize, bitmapSize), 0, 0, bitmapSize, bitmapSize, GraphicsUnit.Pixel);
}

for (var i = 0; i < bitmapSize; i++)
{
    for (var j = 0; j < bitmapSize; j++)
    {
        Color orig = bmp.GetPixel(i, j);
        Color cDrawImage = bmpDestDrawImage.GetPixel(i, j);

        Color examined = cDrawImage;

        if (orig.R != examined.R || orig.G != examined.G || orig.B != examined.B || orig.A != examined.A)
        {
            // My expected result is an exact copy of the source image, so I'm not expecting this
            throw new Exception("Unexpected.");
        }
    }
}