How is it a bug? At no point without doing a /mode #channel +b or /channel does mIRC ever retrieve the full banlist, just the same as it does not fill the IAL without /who #channel or whatever. If you don't fill the IBL then the only thing mIRC will know about is any bans that get set/unset while you are in it, just the same as the IAL works when users join/part/quit.