Players can fly when relogging when they are not allowed to fly

I am running a server using bPermissions and NoCheatPlus. I believe I have my permission levels set up correctly. For some unknown reason, players who are not allowed to fly can after they relog.

After noticing other players doing this on my server, I tried this myself with a player account that is not opt. When I first tried to fly, the server said, "You do not have access to that command." I then immediately logged off then back on. Then the server accepted the fly command. I am not using any mods whatsoever with the character.

What could be causing this problem? Players are flying around my quest world and I can't stop them.