The different OS versions have different repositories. A "pkcon update" does not update anything of the OS unless you reconfigured ssu to point to "latest", which may be as the name suggests a symlink to the latest repo.

Release candidates are dangerous because they may still contain issues that brick devices (or at least require a reset to factory defaults, or some good Linux skills to fix).
For every release candidate that did not become a release, there is at least one critical known bug.

Release candidates generally do not get further update notifications, so you cannot update them through the UI.