Sorry if my question annoyed you. There are several paths that could have been taken. We could have added a WP filter when getting the post thumbnail to replace it with the value of an acf image field or we could set the featured image when saving the field. Either are possible.
Leaving the question here will help others looking for the same thing. Here are some links that explain setting the featured image: https://support.advancedcustomfields.com/forums/topic/use-a-current-acf-image-as-the-featured-image/, https://support.advancedcustomfields.com/forums/topic/set-image-as-featured-image/, https://gist.github.com/kevinwhoffman/0f6b6530564f4aef61fe
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