Just to be clear about this: are you saying that mIRC not saving the log at all? If it's not saving the log, then something is wrong.
mIRC will save the log to #channelname.network.N.log and once that window is closed, it will merge that log file into the main log file #channelname.network.log.
If it is merging the log, then it's working correctly.
If it is not merging the log, can you post a step by step method in a clean install of mIRC that reproduces this issue for you?
