Create a new a channel on any IRC network and set +k, open a new -m server window and try to join the channel without providing the key.

I succeeded to join the channel each time only for the first attempt (for the second and subsequent attempts, it ask for a key). I tried this for random channels and on different IRC networks.

Additionally, after you join a +k channel (whether using the above method or /join #channel <key>) and then leave it, you'll be asked to provide the key if you tried to rejoin again, so it does not look like mIRC is 'storing' the password.

Is this a normal behavior or a kind of bug? If it is a normal behavior, how it is supposed to work exactly?