if the server sends ":nick PRIVMSG target :msg" and the target is not a channel, mIRC interprets this as a private message from 'nick'. so, you're quite right, mIRC will appear to ignore 'nick222' - or, at least, pretend that the message was directed at you.
a well constructed IRC server should not be sending you PRIVMSGs intended for other clients (ie. the target should either be a channel or your own nickname: $me), so mIRC's behaviour does not seem unreasonable. how else do you propose mIRC handle such an event?
the actual target of the PRIVMSG is available to scripters in events such as on TEXT through the $target identifier.
