I make it work in the 4.25.1 .
It looks like you got it to extrude a polygon of a static mesh by polygon index of that mesh, correct me if I’m wrong.
That is awesome it is great to know how at least part of the Editable mesh system works. I will have to try this out myself and see what I can figure out with this new information!
Are you currently using this in a project? Or were you just trying it out?