Adrian,
You'll want to file the issue in prawn-svg. Asciidoctor hands the SVG data off to this component, so aside from reading in the file, there's nothing that Asciidoctor PDF does to draw the image on the page.
Fortunately, prawn-svg has an excellent maintainer who really knows his stuff.
There is an unresolved issue for arrow heads, but I think @mogest is looking for some more input before implementing it, as he explains here:
Cheers,
-Dan