Custom argument not defined in ‘alm_query_args_{id}’ hook

I have the following shortcode,

[ajax_load_more
  id="products_list"
  ...
  custom_args="rating:5"
]

connected to the following filter in a code snippet.

add_filter(
  'alm_query_args_products_list',
  'alm_do_stuff', 10, 2
);

function alm_do_stuff( $args, $id ) {

  if ( isset( $args['rating'] ) )
  {
    // do stuff
  }

  return $args;
}

The value of $args['rating'] in the filter code should be 5, but it’s undefined.