A little more info:
I just tried reversing the two variables ( {mnn3:lc}, {year4} ) and it saves just fine. I also noticed that if I open up the stationary pad and just put {year4}, {mnn3:lc} and click the Apply Stationary to Selected button, all the selected photos will only have the {year4} variable applied.
If I put another keyword at the end like {year4},{mnn3:lc}, family , the whole thing gets applied.
So it seems I can work around it by putting the {mnn3:lc} first.