Auto select free shipping method

We have a free shipping coupon for orders over 150 when the address is in a specific zone. The coupon will auto apply but it does not select the only free shipping method in the zone so customers have to manually choose the free shipping method that now appears. We recall that it did auto select the free shipping method in the past but we can’t recall when so we don’t know when it stopped working. We have verified that the methods, zones and coupons are all setup correctly. How do we get this to work?

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