Possible BigPipe issue

Here is a patch that works for now, but I don't think this is a permanent fix.

A little more context that might help.

We are targeting the tabs in the local-task menu, like this:
.tabs ul li:nth-child(2)

This menu is also manipulated using a hook:

/**
 * Implements hook_menu_local_tasks_alter().
 */
function hook_menu_local_tasks_alter(&$data, $route_name) {

  if (isset($data['tabs'][0]['entity.user.canonical']['#link']['title'])) {
    $data['tabs'][0]['entity.user.canonical']['#link']['title'] = t('Overview');
  }

  $routes_with_tabs = [
    "entity.user.canonical",
    ...
    "user.login",
    "user.pass",
  ];

  $routes_to_unset = [
    "entity.user.edit_form",
    "entity.user.openid_connect_accounts",
    ...
    "user.pass",
    "entity.webform_submission.user",
  ];

  if (in_array($route_name, $routes_with_tabs)) {
    foreach ($routes_to_unset as $unset_route) {
      if (isset($data['tabs'][0][$unset_route])) {
        unset($data['tabs'][0][$unset_route]);
      }
    }
  }
}

"Some routes are removed due to custom routes that are not mandatory for the context."

this could hopefully help for a solution.