Hi @geppie030
If you execute “acf/save_post” with the priority of 10, the image is not processed by WordPress yet. Kindly change it to 20, get the image by using the get_field() function and use the set_post_thumbnail() function to set the featured image instead.
This page should give you more idea about it: https://www.advancedcustomfields.com/resources/acfsave_post/.
I hope this helps 🙂