What I believe to be a bug is the fact that mIRC also opens up the /list window if you disconnect and it hasn't yet received a 321.
The reason for that is that some networks do not send a 321, they go straight to sending 322, if any, and finally a 323.
It can be tricky handling numerics with different behaviours across networks. mIRC attempts to work in a way that is as intuitive and compatible as possible across all of them.
I have added support for numeric 481 so that if mIRC receives it, it will cancel a channels /list request. If anyone is aware of numeric 481 being used for a different purpose on another network please do let me know.