The problem with this file is that only first frame is correct. It will convert the image if you only read the first frame and since you are using MagickImage to read the file this is probably what you want. It is now making me think that this should be something that I could possibly automate. For now you will have to do this:
using (var image = new MagickImage(fullFileName + "[0]")) // orusing (var image = new MagickImage(fullFileName, new MagickReadSettings() { FrameCount = 1 }))