Forums Bug Reports Wrong nickname, and other BNC-related problems.

mIRC uses the nickname specified in the incoming 001 numeric as your active nickname.

mIRC has been in development a long time, and it integrates quite closely with server behaviour, far more than most IRC clients, so if a BNC is modifying server messages in some way, and making assumptions about what an IRC client needs, then yes, that will definitely affect the way the IRC client behaves.