I’m not sure the belt can slip with the snubber in place, and when it slips it should make a popping noise and you should feel it as well. In this case, you can spin freely.
I rather think the ratchet of the freewheel is not correctly functioning. That can be the case if the Strida is regularly ridden in the rain. Water and dirt tend then to penetrate inside and block the ratchet.
Check this topic: [url]My Strida doesn't like cold weather]