Mine have never "auto propagated". I have always had to cut mine to reduce the size. The ones I have seen "auto propagate", I have noticed the polyps are not open like a flower at the ends, like in the picture below. They seem like they are "dropping babies" in an effort to just survive, or are not very happy, as leathers are tough to kill. Bigal, got a close up picture of your polyps? I'm curious to see them. I'm not saying if they auto propatate they aren't happy, but I've never seen one do that, that is happy.
Here is what mine usually look like. Many years, never split on it's own. When it's NOT happy, the open "flower" end of the polyp is usually not open well. Below, it is almost open all the way, but they open even more than what my picture shows. The wider the ends open, the happier it seems. Maybe I'm wrong? Mine didn't come with a manual, just what I've observed...