post__not_in should be an array, so that’s the first suggested change
add_filter( 'acf/fields/post_object/query/name=my_field', [ $this, 'my_filter' ] );
public function my_filter( $args, $field, $post_id ) {
$field; // Undefined
$post_id; // Undefined
if(isset($post_id)) {
$args['post__not_in'] = array([$post_id]);
}
return $args;
}
If this field is a sub field of a repeater or flex field you also need to use the field key for adding the filter.
