@chays77 Based on the information you provided, we expect this to be a bug. We’re actively using the bug tracking on our GitHub repository so your best next step would be to create a new issue for our developers at https://github.com/Yoast/wordpress-seo/issues/new. You will need an account to create a new issue. If this is your first bug report, please check out: https://kb.yoast.com/kb/how-to-write-a-good-bug-report/.
Don’t forget to include the URL to this conversation in your bug report.
@treveres Unfortunately we can’t offer support on custom code. Since we don’t want to take the risk that we break your website by suggesting incorrect or incomplete code, we cannot advise you on how to make such changes. Maybe someone watching these forums can assist you further, but if your topic is inactive for 7 days, we’ll mark it as resolved to keep the overview.
Thank you for your understanding.
We do have additional information on customizing the sitemap using the plugin at the following link: https://kb.yoast.com/kb/how-to-customize-the-sitemap-index/