Diagnosing "Unknown tag 'flowRoot'" warning in PDF generation

I'm trying to move to current versions of asciidoctor & related plugins, and our spec generation is now giving warnings of the form

> asciidoctor: WARNING: problem encountered in image: /tmp/Vulkan-Docs/images/pipelinemesh.svg; Unknown tag 'flowRoot'; ignoring

Running asciidoctor with -v --trace doesn't deliver any more information so I'm unclear where in the pipeline the problem is happening, or if it even is something we should be concerned about. For reference the old / new versions of gems used are:

asciidoctor 1.5.8 / 2.0.10
asciidoctor-diagram 1.5.12 / 2.0.1
asciidoctor-mathematical 0.2.2 / 0.3.1
asciidoctor-pdf 1.5.0.alpha.16 / 1.5.3

Any ideas on tracking this down further? It does not occur during HTML generation, so something about how SVGs are incorporated into PDF generation would appear to be responsible.

Jon

N.b. I can try to produce a minimal case if needed, but for the moment I'm just trying to understand which project I should approach to report the issue.

--

@mojavelinux this may be happening somewhere above prawn-svg. When I do

ruby -r prawn-svg <<EOF
Prawn::Document.generate("test.pdf") do
  svg IO.read("pipelinemesh.svg"), position: :center
end
EOF

(where that's one of the files asciidoctor reports this error on), there's no complaint generated and the resulting PDF looks OK. When I try to run asciidoctor-pdf on the actual document, though, these complaints pop up. So maybe asciidoctor-pdf is the right place to file an issue?