As I understand it the data do not tell us anything directly about the propensity of JW couples to divorce, because it doesn't tell us 1) if the person divorced while a JW or 2) whether both partners were JWs. It could be that divorcees join JWs in higher proportion. And it could be that many JWs divorce non-JW partners.
In my observation many JW marriages end when one person (usually the man) stops being a JW and the woman continues.