Welcome! I'm going to just be talking through my hat about this but here's a few thoughts.
Beautiful flower. That sort of thing happens often in my dahlias. Perhaps it is "senescence" - a word I learned not all that long ago. A doctor might use the word "maturity" but I wonder
. My idea is that genetic traits are not expressed consistently over the years and with cloning, some varieties of dahlias and roses must have quite a history by now.