Magick.NET 7.0.0.0017 has been release and includes an option to prevent this behavior. If you set the attribute "colorspace:auto-grayscale" to "false" the automatic conversion to grayscale won't happen.
using (MagickImage image = new MagickImage("Input.jpg")) { image.SetAttribute("colorspace:auto-grayscale", "false"); image.Write("Output.jpg"); }