ref tags don't work if the ref is pointing to a label in another document
The current ref implementation seems to only work with labels in the same document, while Sphinx allows refs to point to any label in the documentation.
I just encountered this as well. Very frustrating.
As far as I've been able to tell, the only way to get this working would be to connect to the Confluence API and query for the server path of the pages ahead of time. In other words, it's not possible for the builder to produce accurate links because it has no knowledge of where the link should point to.
It may be possible to avoid needing to connect to the API for every build if this information is provided in the config.yml file. Perhaps the conf_page_maker could be augmented to add a server_path attribute that can be used to build appropriate links.
FYI, I've got a fork that mostly works with cross-doc references and a couple other features.
Would be happy to contribute back, but looks like this is unmaintained.