Sitemap.xml when xhtml:link used, displays in browser as plain text

[AdamczykMaciej]

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Seotoolkit Sitemap
When xhtml tags are created in my sitemap.xml then it appears as plain text in browser. When inspecting the source code it is a valid xml but in browser it looks like a plain text. I found similar threads:
https://stackoverflow.com/questions/60564479/sitemap-xml-with-xhtmllink-rel-alternate-hreflang-href-for-multilang
https://stackoverflow.com/questions/51923627/xml-sitemap-rendering-as-plain-text

I think it might be the issue with
https://stackoverflow.com/questions/24864254/google-multilingual-sitemap-issue