When using Kunena 1.6 and greater GJ uses KUNENA API to add create, update, delete, etc.. regarding forums. Their API should add the user if they do not exist, but it does not.
Kunena user table should keep in sync by acting on Joomlas triggers (register or login, these are still fired even by logging in via CB). It's unrealistic to expect any extension to do the sync up FOR Kunena. I will be discussing with the Kunena team for a better solution.
You would have issues for instance if you login, never visit forums, edit profile and attempt to add a signature. This would cause the same problem. As in it wouldn't update.
[UPDATE]: Have communicated with Kunena team and will remain in its current state until Kunena 2.0 where everything is properly handled.
Will be implementing code to ensure user table is in sync and removing all API usage until 2.0. Bug ticket as follows.
forge.joomlapolis.com/issues/2457