🇺🇸United States mpifalo
Actually, this issue is caused by a patch we added for core: https://www.drupal.org/project/drupal/issues/2988123 🐛 Appending/removing a field item to a field item list doesn't notify the parent of the change Needs work . So this is not anything to do with EntityQueue. Will mark as closed/won't fix - sorry for the noise.
🇺🇸United States mpifalo
More info: we can trigger the error with custom code in our theme $items = $subqueue->get('items')->getValue();.
Created a patch locally and am testing up in our dev environment; if it works well I can try to do a merge request if no one else has done so yet.