I've been playing RS2 during the Double EXP event, and I hit a new level, only for the next game to put me back to the previous one. Closing to menu after a game also would show the previous level, as if I never leveled up. This happened multiple times in a row, until I restarted the game. I'd hit the level, then next game be back where I was. Rinse and repeat. I lost about 4/5 levels worth of experience because of this.