We managed to fix it using a different approach.
We re-added the group and fields in the ACF setup, being careful to name the fields the same as what they were named before. The fields then reappeared in the Woocommerce Product records, and the data reappeared in those records that previously had data in the fields.
Not sure what made the field definitions disappear in the first place, but I am suspicious of the latest plugin upgrade.
