dblatex (via gtkdoc-mkpdf) warns that labels must be integers:

    Warning: only an integer in @label can be processed: 'I'
    Warning: only an integer in @label can be processed: 'II'
    Warning: only an integer in @label can be processed: 'III'

Make the labels decimal to placate dblatex. I'm not sure this is
actually the best solution, though.

https://bugzilla.gnome.org/show_bug.cgi?id=786174