For the record, the values used for candidate composes seem to be based on the label (which is a thing only candidate composes have, not nightlies). E.g. for the Beta we got 31_Beta.1.1 as the version string. This is also odd and not in line with productmd; the actual label of the compose is Beta-1.1, not Beta.1.1, as you can see at https://kojipkgs.fedoraproject.org/compose/31/Fedora-31-20190911.0/compose/metadata/composeinfo.json .
Edit: also, why on earth does the string start (version).(date) for the nightlies, but (version)_(labelname) for the candidate composes?! Why a dot for one case but an underscore for the other? I think I need to go sit down with a warm cup of tea.