Thanks for your bug report. The behaviour you describe is intentional. If a network name is not provided, there is no network name to use in the log filename or for creating a folder.
With your suggestion, if a user had multiple servers defined for a network but had forgotten to assign a network name, the opposite issue would occur: the log files for the same network would be split across multiple files and folders and the user would have to figure out which log files belong where. If the user connects to multiple networks, that could mean tens of files or folders with fragments of logs.
This feature depends on the user specifying a network name for the server.
That said, it is actually possible for log files to intermix in other situations as well. To get around this issue, many long versions ago, mIRC included a change to prevent this (by creating unique log filenames with id numbers per server connection). Unfortunately, users disliked the feature as it made keeping track of log files in real-time, eg. using scripts, too complicated. So the feature was subsequently removed.
