The isosurface itself is fine (also for Bloch states), it's only the coloring (of complex grids, like Bloch states) that's wrong.
If you need to open an NC file from 13.8 in 12.8, you should open the file in a proper editor (so, not Notepad or something silly like that, but "vi", "emacs", or "notepad++", etc), and find the version number, "13.8". Just change it to 12.8 (make a backup of course of the file first), and then try opening it in VNL 12.8. Don't use search and replace since there may be other instances of 13.8 in the binary data in the file.