I was unable to reproduce a problem I'm afraid. I connected to iba.bynets.org (unrealircd 3.2.4), joined the largest channels, and clicked on the bans/invites/excepts buttons, opening and closing the channel central, and parting/re-joining channels. It all worked fine.
I also had the a debug window open and that showed no anomalies, the server correctly returned the bans/invites/exceptions lists each time...
Have you tried testing this with a newly installed, clean copy of mIRC, installed to an empty folder? Is it possible that you may be using a script that is causing the problem?
