Not really sure if this is a bug here, or in the mercurial plugin, but the MercurialScmAdapter gets the revision id by calling hgSCM.getRevision(), which just returns whatever value set in the job configuration and defaults to "default". Bitbucket accepts this, and I'm not sure if it actually tags a commit somewhere, but it returns a url I can't access.

I looked at the code briefly, and it looks like the only way to get an actual revisionId is to call
one of the versions of hgSCM.calcRevisionFromBuild(), cast that to a hgTagAction and get the id of that.