Thanks! Sadly I can't replicate the issue but have a workaround. RPGM keeps a list of saves in the global save file in save/global.rmmzsave.
If the saves aren't recogniced you can: Backup your save folder -> open the game and create a dummy save in the save slot you want to load -> copy&paste the backed up save back into the save folder. You should then be able to load the save.