It appears that for whatever reason, Google suddenly is choosing to index some alternate version of your URLs which isn't the one that you list in your sitemap. This is unlikely to hurt your site or your SEO. As long as some version of each page is indexed, I wouldn't worry about it too much.
That being said, you should check that you didn't make recent redirect or canonical tag changes that conflict with your sitemap. Your XML sitemap should list your preferred URLs. Those URLs shouldn't redirect or have canonical tags that point to other pages. If you have redirects or canonical tags for
www, make sure that the XML sitemap agrees with them.
You should also check that your site uses the canonical links in all internal links. Have alternate URLs in internal links can also cause Google to index different URL versions.
On the other hand, Google sometimes makes weird decisions about which of your URLs are canonical, and there is not much that you can do about it. Google doesn't always index your preferred canonical URLs, especially right after making changes. It can take Google many months or even more than a year to come around to agreeing to index the URLs you choose as canonical, even if all your signals (sitemap, redirects, canonical tags, and internal links) are in agreement.