Sub-fieldset inherits from the "collapsible" property of parent

I've noticed a problem when you create a non-collapsible sub-fieldset within a collapsible one. The sub-fieldset remains collapsible as its parent. I've tried to reproduce an example of a buggy code:

  $form['parent'] = array(
    '#type' => 'fieldset',
    '#title' => t('Parent'),
    '#collapsed' => TRUE,
    '#collapsible' => TRUE,
  );
  
  $form['parent']['child'] = array(
    '#type' => 'fieldset',
    //'#title' => t('Child'),
    '#collapsible' => FALSE, // XXX: this does not work
    '#collapsed' => FALSE,
    '#description' => t('Child')
  );