I suppose it could be, but I hope someone else who grows Neos will chime in here. I'm thinking the cause(s) for something like this could potentially include too little light, too much fertilizer, too much warmth and/or moisture at times when the plant would prefer to be taking a rest, or the use of certain chemical fungicides. I think it's entirely possible that the bad effects of cultural missteps might not surface until several months after the fact.
Also, with proper culture your plant could resume production of properly variegated leaves.
I'm assuming you are talking about the main/original portion of the plant and not a sport or keiki mutation coming up alongside the main part of the plant. It's also important to know that sometimes new babies start out with little or no variegation and develop the proper variegation as they mature.