hook_form_alter makes js on every other form not work

Question

I use hook_form_alter() to alter a field.

function mymodule_form_alter(&$form, &$form_state) {
  if ($form['#id'] == 'my_form') {
    $form['my_field']['#value'] = 'New value';
  }
}

It changes the field value to new value, but the problem is that, as soon as I enable this module, the JavaScript on forms stops working. For example drop-downs, autocomplete, AJAX no longer works.

I cant understand what cause this.