Same here. Mine self-sow moderately and usually in welcome places. They all look the same as the original I planted more than 20 yrs ago.
I'm with Quercusnut here. I planted a few and they have been moving around to the places they pefer for 10 years now. The spread is definitely from seeds and the plants that naturally grow from seeds seem the same as the originals. They are native here in MD. That probably helps.
My original planting of a 6 pack have moved around to 4 new locations and I can almost bet those are the sunniest locations in the yard. I probably have 4 dozen now. Transplanting them is not a good idea. You won't find a better location for them than they will themselves.
Every transplant attempt fails. I can't quite guess right about where they want to be. But THEY know! Their seeds somehow move around and when in the right spot, you will find a dozen next year!
Or you don't have good spaces for them.