They're not "the arms of Juan Carlos I", but those of Spain after 1977. Imo it's just a red colour fąding. If it was red colour missing, the spaces suposed to be red, should have been white, which is not the case.
I thought is was not a missing red overprint however I looked closely at the correct stamp on left as well as the other eight examples I have and the red is printed on top of the yellow. So I am certain it is not fading.
Red-missing stamps are the most common color missing error candidates we see here. Red is at the end of the visible spectrum - we all learned the ROYGBIV in grade school. Red is the most susceptible color to sun-fading. Used stamps are also more problematic to "prove" color-missing varieties. Who knows what the stamp has been exposed to during the past 36 years. Almost any red-missing used stamp will be hard to get a good certificate for. It has a very high likelihood of being a changeling due to light or chemicals. Try to get a good certificate if you think this is a true color-missing error.