This can be done in other ways.
Create an acf/pre_save_post filter https://www.advancedcustomfields.com/resources/acf-pre_save_post/, and move the image to the post thumbnail. There is an example of this here, search for “_thumbnail_id” on that page
Create an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/, there is an example of this here https://itsmereal.com/advanced-custom-fields-front-end/
Create an acf/update_value filter https://www.advancedcustomfields.com/resources/acfupdate_value/, there is a solution for in this topic https://support.advancedcustomfields.com/forums/topic/set-image-as-featured-image/
But the easiest method, that most people overlook, is to create an image field with the field name of “_thumbnail_id”. Doing this ACF will just store the image in the field it needs to be stored in and in the same format. No extra work required for this one.
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