GetGame().GetBackendApi().GetPlayerUID(playerId) is not returning the correct playerId in the Workbench. Both with or without playing on a localhost server.
It would be nice if the ID is returned from the BI account that is currently signed into the workbench. It could return the same ID for all peer clients you connect or intentionally offset their GUIDs by the peer number so their hash is somewhat "unique" but still deterministic based on the main Workbench's BI-ID.