Hi, thanks a lot for your research.
To be honest, I'm not working with Getty (yet). I only use this free field because it appears in PM and C1 to make some extra notes for Instagram profiles of persons i photographed. I don't really want to use any other free IPTC fields, but maybe this would be the workaround. The IPTC Subject Code is properely synced between PM and C1. Anyway, I will ask Capture One why they haven’t implemented this Getty field properely.