Error was not related to the sitemap itself, but an issue with empty links in this implementation which were appearing in the sitemap, hence the gist of my original question.
So, this is a browser issue? Hmm…The browsers are unable to output the document tree for the multilingual sitemaps, so checking the source code is the only way to view the tree properly.
Will test this and see if it works. I liked it back when I used WP I could style the sitemap. I have had a few requests to do it for a site I co-author with friends that I migrated to Hugo.