Kunena doesn't see users who for some reason have no saved profile.
This happens for example in userlist. Also link to the user is missing, for example in last member link (in statistics).
Fix #1974: Kunena doesn't display users who aren't in #__kunena_users…
Easy way to test this: add some users into Joomla, but don't log in with those.
It works on my test pages correctly