#limit_validation_errors seems ignored inside custom validation function when used on AJAX button

Site with only mymodule and views (with dependences) enabled beside core modules.

If my validation function looks like this:

function mymodule_form_validate($form, &$form_state) {
  // ...
  form_set_error($element, $error);
  // ...
}

errors are set during ajax call even if the button in question has

      '#limit_validation_errors' => array(),
      '#submit' => array(),

However, if I write it like this:

function mymodule_form_validate($form, &$form_state) {
  $limit_validation_errors = $form_state['clicked_button']['#limit_validation_errors'];
  // ...
  form_set_error($element, $error, $limit_validation_errors);
  // ...
}

it works just as it's supposed to.

Am I doing something wrong, or is there a bug somewhere?