If I understand you correctly, it looks like you are trying to get the choices in the post object to display in alphabetical order.
Have you tried the following:
function my_post_object_query( $args, $field, $post_id ) {
// modify args
$args['orderby'] = 'title'; //title of the post
$args['order'] = 'ASC'; //order a-z; use DESC for z-a
// return
return $args;
}
add_filter('acf/fields/post_object/query/key={field key}', 'my_post_object_query', 10, 3);
Change the field key to the field key for your post object (it should look like this: acf/fields/post_object/query/key=field_508a263b40457 . Then add the code to your functions.php file
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