Scanning a huge log directory is very resource intensive. mIRC only looks to see if there is a previous log of the exact expected name, and if that file does not exist then mIRC gives up on log playback.
If mIRC were to scan your huge log directory for the last-known most recent log from that user or channel, then it can spend upwards of 5 minutes scanning through files and mIRC would be completely unresponsive for that time period.
What you are asking for would require mIRC to maintain a database of logs and references to the most recent log of every channel / query / network entity. A log of logs, so to speak.