I've added some diagnostic output for when stats are being loaded. You should something like this when you connect:
[STATS] Loading your personal bests...
[STATS] Your personal best speed has been loaded!
[STATS] Loading your rank...
[STATS] Your personal rank has been loaded!
I think the cause may have been a timing race condition for players who load into the server quickly, and I've adjusted the plugin a little to correct for that. Let me know if any of that helps and the issue goes away.