I planted a gorgeous iris last year that had striped leaves. However, when it came back this year the leaves were plain green. What happened?
NEVER! It is like saying that humans can change colors.
What has happened is one of two things, either overcrowding and the more dominant one took over and killed the lesser one, or seeds developed somewhere and either dropped in your garden or critters including birds will bring them in and drop them in.
Hope this helps.
The leaves can change to sold green if it doesn't get enough light. I have had this happen on other variegated foliage. I also have had the flower shade be lighter or darker depending on how hot it was when it bloomed. Navajo Jewel last year had no hit of blue because it bloomed when it was hot, but this year it was a very light shade of blue because it bloomed before our hot weather hit.
I imagine the variegated gene is still in the rhizome, and that the next set of leaves may be striped again if it gets the right light. Sometimes whole branches of a variegated shrub will revert to the solid color, for some reason. Maybe it's the same with irises.