mIRC does not perform a /who #channel when you join a channel as IRC networks will often disconnect clients that perform commands that exceed a client's sendq (the maximum amount of data that is allowed to be pending out on connection). Each IRC network has its own sendq limits. This means that a scripter should only issue a /who #channel if it is needed. A /who #channel is the only way to ensure that the IAL is filled for a particular channel. For a large channel, you may need to create a script that fills the IAL slowly or that fills it when it is empty for a particular user.