asciidoctor-pdf: asciidoctor-diagram generated svg is searched in wrong path

In my project root I have a Gemfile with:

namespace :book do
    task :build do
        backends = ['html', 'pdf']
        backends.each { |b|
            Asciidoctor.convert_file 'book.adoc',
                to_dir: 'dist/book/' + b,
                mkdirs: true,
                safe: :safe,
                backend: b
        }
    end
end

asciidoctor-pdf yields:

WARNING: image to embed not found or not readable: <projectdir>/<file>.svg

I think it should be resolving the path like this instead <projectdir>/<to_dir>/<file>.svg?

The svg is properly generated in <to_dir>.

Thanks.