When exporting an SVG from the extension in VS Code "-svg" is appended to the piece id. e.g. the resulting file name is of the form <process id>-svg.svg. The SVG isn't then detected for display unless the filename is edited.

After some discussion:
Re-reading the documentation I see now that it says "For example, when the SVG files are generated, rename the files to {{

-svg.svg }} and it's not obvious why it would be necessary to rename.