I tried the above shape and got the same problems/results. I also tried using a free envelope (and all the others), but, that didn't work either. You should not have to use any extrusion envelope at all on this spline object anyway. I think you found a bug. If you find a way to make the shape work properly please share. My bet is that a free extrusion envelope will not work for you either. Good luck in finding the answer. I hope it works for you.