You should post debug output including raw 005 from the initial connection (this is where the server should be telling mIRC which modes take parameters) and also the debug data from joining a channel, in particular raw 324. Without these there's no way to know whether mIRC is at fault or the server.
Spelling mistakes, grammatical errors, and stupid comments are intentional.
Thanks for your bug report. Currently mIRC only stores the values for +lk and does not store custom modes added by different networks. This will require a big change to the way channel mode settings are stored. This is on my to-do list.