Why the insert?
The spring eyes are handformed, they could have made them to fit the 35mm bushings just as the old springs did, however it was decided that by using an insert, the bushing would have a much more consistent surface to ride against, as well as be less likely to be damaged by the spring itself. According to the tech, it is pretty uncommon to have sleeve move, but the fix is easy. If you can pull the sleeve all the way out of the spring, do so, and slightly "crush" it in a vice to make it an elongated shape (very slightly). Then, press it back into the spring, this should hold it for good. If it moves a bit, but not enough to completely remove it, use a punch to "stake" it into the spring, the best location being right where the main leaf eye termintates as there is a small gap there.