Linda,
Serenade self sows in my garden and it has not changed in the 4 years I've had it. It is at least 60 feet from any other MG's which reduces the likelihood of cross pollination and MAY be why it is stable for me.
Did you happen to get the seeds from me? Mine are as they should be this year but I think there may be some environmental factors that influence form in the hige types, but I am far from an expert. This is just my experience in my garden.
Alana

Linda it is possible for them to revert back but you should be seeing hige like flowers too, not all singles.

My SS has been reseeding true for several years. I occasionally get a bloom that's not split but it's rare.