Wrong destination when editing the original language translation

If the Webform module doesn’t handle the destination properly, you could override the destination parameter temporarily in a custom module or by using hook_form_alter:

function custom_module_form_webform_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if (\Drupal::routeMatch()->getRouteName() == 'entity.webform.canonical') {
    $form['#action'] = '/admin/structure/webform/manage/' . $form['#webform']->id() . '/translate';
  }
}