@derevnya
We can see the sitemap is loading and showing the content as expected. We would expect Google would index it. We are not sure why it is failing. We suggest the following actions:
1. Clear all your caching from your theme, plugin, server, CDN like CloudFlare or browser. If you are not sure how to clear caching from theme/plugin please speak to those authors. If you want to clear cache from the server, please speak to your host provider. To clear the cache from a browser use this guide:?https://kb.yoast.com/kb/how-to-clear-my-browsers-cache/.
2. Delete your sitemap in Google. Doing so will not hurt the SEO of the site. This guide explains more (scroll to the bottom):?https://support.google.com/webmasters/answer/183669?hl=en.
3. Submit the sitemap again. Be sure to submit only sitemap_index.xml and nothing else.?This guide explains more:?https://kb.yoast.com/kb/submit-sitemap-search-engines/.
4. Monitor the results to see if the issue resolves and pages get indexed.