Jumping in here....
First, I do not grow any of the larger sedums myself, but have some I follow in the neighbor's yard. I do have a couple of thoughts to share and an experiment to try.
Some thoughts come to mind 1) Perhaps a portion of your flowers are not visited by pollinators, resulting in a non-fertile seed body area that simply dries up. Try to see if the spot starts after the flower opens and stays open for awhile, or if the spot appears before the flower opens. 2) In areas with intense sun, water on a flower or plant can focus and intensify the sun's heat, resulting in burning. So does your watering routine leave areas of water on the plants when the sun is intense? Do you water in the middle of the day?
Here's an experiment to try. Place a zip lock bag over the flower head before the flower opens. Hold the bag in place with some tied soft thread and zip up as much of the bag as you can. Watch it to see if any spots develop. That may tell up something.
