For this to work, person must access registration page using linkage (slug) that is associated with the group (not just by using registration instance id). This will also result in person being automatically added to the group with the appropriate attributes being set as soon as they register. This does not work when using Group Placement.
You would need to add a slug value to the linkage and then access the registration using URL like:
https://rock.cfbc.org/Registration/SlugName