I don't think anyone has reported such a bug before and I haven't experienced it myself, however, I don't believe I've attempted to /list 10,000 channels since 6.16. This command relies a lot upon the server, so try /listing on another network and see if that helps. Different networks can have different methods of relaying the list to the client. Try using /list on DALnet, it's speedier than other networks, and there's easily over 10k channels at all times.
As usual make sure you test this on a FRESH install of the latest version of mIRC and you're only typing /list with no other parameters.
Regards,
