Set featured image from custom field URL

Hook 'acf/save_post' after ACF saves the $_POST['acf'] data. Then check to see if the post has_post_thumbnail and set_post_thumbnail if it doesn’t.

function save_acf_image_to_post_thumbnail( $post_id ) {

    $image = get_field( 'fl_image' );

    if ( ! empty ( $image ) ) {

        if ( ! has_post_thumbnail( $post_id ) ) {

            $image_id = isset( $image[ 'id' ] ) ? $image[ 'id' ] : ( isset( $image[ 'ID' ] ) ? $image[ 'ID' ] : '' );

            if ( ! empty ( $image_id ) ) {

                set_post_thumbnail( $post_id, $image_id );
            }
        }
    }
}

add_action( 'acf/save_post', 'save_acf_image_to_post_thumbnail', 20 );

This assumes the Return Value is an Image Object.

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It’s also possible to trick has_post_thumbnail and post_thumbnail_html to render your custom field Image URL when the metadata is missing.

Spoof has_post_thumbnail:

function filter_post_metadata( $value, $object_id, $meta_key, $single ) {

    if ( $meta_key === '_thumbnail_id' && ! $value ) {

        return empty ( get_field( 'fl_image' ) ) ? $value : - 1;
    }

    return $value;
}

add_filter( 'get_post_metadata', 'filter_post_metadata', 20, 4 );

Doctor post_thumbnail_html:

function filter_post_thumbnail_html( $html, $post_ID, $post_thumbnail_id, $size, $attr ) {

    $image = get_field( 'fl_image' );

    if ( ! empty( $image ) && (empty( $post_thumbnail_id ) || $post_thumbnail_id === -1) ) :

        $html = sprintf( '<img width="https://wordpress.stackexchange.com/questions/242671/%s" height="auto" src="https://wordpress.stackexchange.com/questions/242671/%s" class="attachment-post-thumbnail size-post-thumbnail wp-post-image" >', "100%", esc_url( $image)  );

    endif;

    return $html;
}

add_filter( 'post_thumbnail_html', 'filter_post_thumbnail_html', 20, 5 );