I think you may be right.
The problem is that you need the attachment/image ID in order to set the post thumbnail.
But instead of do_action, in your pre save post function change it to:
add_action('acf/save_post', 'tsm_save_image_field_to_featured_image', 20);
I’ve set the priority to 20 above so it runs after the acf save post action and ACF has saved all of the custom fields.
function tsm_save_image_field_to_featured_image( $post_id ) {
// this function will only be called if the pre_save_post
// filter was run because that's where we added the hook
// so you can remove all the tests
// I am using get_post_meta here to make sure
// we get the ID of the image field no matter what you have
// set the image field to return (URL, ID or Object)
$attachment_id = intval(get_post_meta($post_id, 'your_image_field', true));
if ($attachment_id) {
set_post_thumbnail($post, $attachment_id);
}
