The experience for Mining Drills seems to be resetting its count whenever any player goes in or out of any warp level in our multiplayer game.
Looks to reset when any player uses platform stairs, or when going through the harvester teleporters.
I've been checking xp count repeatedly on a miner and seeing it go up, and my friend goes through a teleporter and the xp resets to 0.
Unsure if this is multiplayer exclusive or not just yet.