I just updated from 2.5.4 to 2.5.6 but the issue persists as you stated. I'm not really sure what they changed to confirm I have the change outside of seeing 1.8.1 and ran all of the CB DB tools.
Do other members of your site see regular users as administrator or just the real admins?
Because the problem I was having was that as soon as a new user posted in kunena I would see in there profile the "Administrator" status but the strange thing is only other real admins could see it, as soon as a regular user would look into a profile they would see it as "Fresh Boarder" the way it should look.
I noticed the same thing in the beginning but now it seems to be the same no matter what. I have verified that some accounts show as Administrators when I am logged in as an admin, registered user and logged out.
That is strange that you are still getting the same thing. I just verified that its fixed for me just to make sure my eyes are not playing tricks on me and everything seems fine. I wonder if the problem is still there for the other users?
I never did this but maybe clear the cache in Joomla and clean out the cookies of your browser and see if the problem is still there.