This is just a characteristic of some rhodendron hybrids inherited from one or more of their wild ancestors. I have never seen it in Chionides, but it's common in Cunningham's White and several red flowered varieties. It is unusual to have happened every year for 10 years as weather factors influence fall partial bloom and it does not happen every year in my experience.

Where are you that it is currently fall, but you are growing this plant? I thought it was rather old fashioned and likely to be superseded by more recent varieties, especially in places that are zn 9 & 10. (which is true of most of the civilized/settled parts of the Antipodes)

Maybe Argentina?