I remember that Jomsocial calls the "User created" event before filling his user info in Jomsocial tables.
So, when Joomdle calls Moodle to create the user, Jomsocial profile info is not yet available.
Thus, first time the user logs in after registration, his details won't be available in Moodle, until he saves Jomsocial profile again.
I guess you could fix this behaviour forcing the profile sync on user login, so when the user logs in after registration, his moodle profile is synced from Jomsocial.
To try this, add:
JoomdleHelperContent::call_method ("create_joomdle_user", $user);
in function onLoginUser, in /plugins/user/joomdlehooks.php