I know this is an old topic, but after doing this for some time I’ve just recently realized that using a filter isn’t needed. All you really need to do is create an image field with the field name of “_thumbnail_id”, and ACF will just update the featured image for you. There’s no error checking, but simply making your image field required deals what that problem.
Just thought I add this for anyone that finds this topic.
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ACF PRO’s Flexible Content field allows you to create smaller pieces of UI and compose them into whole flexible patterns. In our latest article, we show how to use it to create swappable site sections and integrate it all in a theme.https://t.co/ZRocH8oJSp