If its the O/E spare wheel carrier it sounds like you are not winding the spare wheel all the way up - when it gets to the top the bit on the end of the cable fixed to the wheel should slide on to a ledge so the weight is not on the winch wire, Is there an obstruction stopping the wheel going right to the top?
Years ago there were stories of spare wheels disappearing off the X2/50 spare wheel carrier. I don't know whether the spare wheels were falling off or getting nicked. But some owners complained and Fiat did a recall and fitted a modified but very similar carrier. Have you got the old type that is suspect and was recalled? Also have you got the right carrier - the bit on the end of the cable you attach to the wheel is different for different size wheel centres.
Easiest solution if you have access is to put a long nut and bolt through one of the wheel bolt holes and through the van floor with a nylock nut on the inside of the van. (I use an 8mm eye bolt I got from an agricultural merchant as farmers use in wire fencing) Making the spare secure both from thieves and from winding itself down and falling off.
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