The SLD isn't directional. If it slips during engine braking, I would think it would slip at anything more than about half-throttle when accelerating. They usually make quite a racket when they slip.
The clutch will occasionally disengage on our '15, allowing the machine to freewheel when running down hill. A quick blip of the throttle will re-engage the clutch and resume engine-braking. Could this be what is happening?