Emmanuele Bassi commented on a discussion:
It's likely because libhandy uses Meson, and library-web does not know how to handle modules that have no documentation inside their release tarballs. You'd have to configure library-web to look in another place for the libhandy documentation archives, and then upload the documentation separately from the release archive.
Ideally, library-web would just go away, and be replaced by a remote location managed by gnome-build-meta, but for libraries that are not part of our SDK, having the documentation available in the project's GitLab pages is perfectly fine.