Well, when you say "reconnects" do you mean when you press the disconnect button and then press it again to reconnect? Because with v6.11 it reconnected with the same port.
If you mean you disconnect and then type /server irc.server.here and it connected with 6667, that's because mIRC takes /server to be a new command and doesn't remember the past input ports for that particular server.
I tried it with acool.dal.net, port 7000, v6.11, Win XP Pro.
It should however, remember the port you last used and the server you last used if you have "Connection on startup" checked, and should reconnect to the last specified port, not 6667 (unless 6667 was the last one you used).
I don't think it's a bug though...unless I'm not understanding what you've said, in which case, please repost

Happy chatting!
Regards,