I don't know much about stem cells at this time, but I do know they frequently try to coax them into the type of cell they want them to become before they inject them. I think a lot of things can affect the severity of the myotonia, but I couldn't even begin to speculate in this instance, except to say, maybe it shifted something with your biochemistry.
You might try e-mailing some researchers. I doubt they will have much of an answer but they might find it interesting.