According to OP in the discussion comment, we know that the .Net platform is on Android.

Let's remove some "noises" from the original code:

// this tells MAUI how to interpret the stream, in this case its BMP
IImage image = PlatformImage.FromStream(await results[0].OpenReadAsync(),Microsoft.Maui.Graphics.ImageFormat.Bmp);
// from now, image is internally just a 2 dimensional RGB array and has nothing to do with image format

// resize the image, still not related to image format
image = image.Resize(320, 480, ResizeMode.Bleed, true);

// .AsBytes() ignores requested format if the encoder doesn't support it and fall back to default PNG format.
// But why???
byte[] imageData = image.ToPlatformImage().AsBytes(Microsoft.Maui.Graphics.ImageFormat.Bmp, 1);

.AsBytes() is an extension method which works on any class which implements the IImage interface.

In Microsoft.Maui.Graphics.ImageExtensions static class it is defined as such:

public static byte[] AsBytes(this IImage target, ImageFormat format = ImageFormat.Png, float quality = 1)
{
	if (target == null)
		return null;

	using (var stream = new MemoryStream())
	{
		target.Save(stream, format, quality);
		return stream.ToArray();
	}
}

GitHub source code:

• https://github.com/dotnet/Microsoft.Maui.Graphics/blob/main/src/Microsoft.Maui.Graphics/ImageExtensions.cs

So it is calling the actual class's .Save() member function, which class implements the interface IImage.

For Android platform, it is the class Microsoft.Maui.Graphics.Platform.PlatformImage from this source code:

• https://github.com/dotnet/maui/blob/main/src/Graphics/src/Graphics/Platforms/Android/PlatformImage.cs

The member function is defined as such:

public void Save(Stream stream, ImageFormat format = ImageFormat.Png, float quality = 1)
{
	if (quality < 0 || quality > 1)
		throw new ArgumentOutOfRangeException(nameof(quality), "quality must be in the range of 0..1");

	switch (format)
	{
		case ImageFormat.Jpeg:
			_bitmap.Compress(Bitmap.CompressFormat.Jpeg, (int)(quality * 100), stream);
			break;
		default:
			_bitmap.Compress(Bitmap.CompressFormat.Png, 100, stream);
			break;
	}
}

That means if the format parameter is not JPEG, it always falls back to PNG, including your specified BMP.