This plugin says it saves levels in case a player leaves and restores them if the player reconnects to the server, so it might be able to help with just the resetting you are having. I haven't tested it, so you would have to do that yourself. Additionally, you may have to create a new config for whatever the most recent version of minas is, since this plugin was last updated 3 years ago (though you might get lucky if the mapper hasn't changed how the leveling system in it works, in which case you would just need to rename the config file).
https://github.com/Locomotivers/sm-p...ster/SaveLevel