I could very eaily see Jan (or Hubertus) Van Eyck painting the lamb the way it is shown despite them both being masters of their craft as well as knowing perfectly well what a real lamb looks like. The symbolism of the day was often more important than fidelity to reality.
I think post 51 has got it. Some later painter, fancying himself more technically able, decided to repaint the lamb more ‘realistically’.
I think they have ways of ‘x-raying’ old paintings to see if there are pictures underneath (terahertz spectroscopy); but I wonder if they can tell how much later a layer of paint was added over an original...