This is a little bug, but I wish it was fixed.
For some reason selecting, re-extruding (using extrude surface) a poly-surface does not create a lightweight extrusion result.
I can copy the edges of the face and that itself can be made into a lightweight extrusion.
BTW, I was curious has anyone made a utility to globally replace all eligible surfaces to lightweight extrusions? It would seem doable to check for poly-surfaces with 2 parallel similar faces, and recreate the extrusion.
Thanks for looking into it,