First of all, /mnick behaves EXACTLY the same way as /nick does once you're already connected to a server (it doesn't give you a "* Your main nickname is now nick" reply and it changes your current server nickname). I don't know if this is a bug or not, but it certainly defeats the purpose of having a /mnick command (which I think should change your main mIRC nickname without changing your current server nickname when you're connected).
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If, at any moment while connected, you type /mnick <your current anick> or you use /tnick and the commands are successful with the server (you get a reply with your new nickname), /mnick will now always work the same way it does when you're offline (sending a "* Your main nickname is now nick" reply), and /nick will no longer change your main mIRC nickname (as it normally does), no matter how many times you use it and the server sends you a reply with your new nickname.
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The "Preserve nicknames" option is disabled, BTW.