Can you describe the steps your taking which are failing? Because I was able to see logs from February during March by doing the following:

1. enable logging, selecting the option to date filenames by month
2. make sure the option for "automatic reload" is checkboxed for "chat"
3. copy into the logs folder, and give it filename: othernick.NameOfTheNetwork.20210201.log
4. Restart mIRC so there's no question of whether it needs to be a continuation of February, then reconnect to network
4. have the other nick send a message query to me

This opened a query window from the other nick, with a couple dozen lines from that February logfile. Though these reloaded lines aren't being redundantly logged into the March file too