Hey Jonathan,
when I apply Mirror modifier it seems to add an extra wireframe on top/outside of the skin modifier. Resulting that when I move the mirror modifier up as instructed there are no changes to the extruted top vertex
For some reason it seems to be sorted. I spent hours on this yesterday to try resolve it. When I started a new project and tried again it was resolved? Basically, once applied the mirror modifier the Pane looked as if it duplicated. This was probably within my own error.
Pretty quick reply man, thanks.