I've just seen a YouTube video by Mejash that explained how to hold more than 30 replays.   In the latest patch a change was introduced that turns on battle replays by default (previously this had to be done manually).  Part of this change was a default limit to the number of replay files held to 30.  Previously, I don't think there was a limit.   This is configured in the \RES\engine_config file - seach for "maxreplay" and change the value from 30 (I've changed mine to 200).   So, you may have lost some replays without knowing it.   Yes, I don't read patch notes...