The RBAC web UI doesn't add service accounts to brand new groups. This happens in both prod and stage at https://console.redhat.com/iam/user-access/groups/add-group . Illustration:

A service account can be added to a group after-the-fact, from the group details page at https://console.redhat.com/iam/user-access/groups/detail/$group_id/service-accounts.