Google updated the Core Web Vitals report in Google Search Console on March 27, 2023. The update may have resulted in a change in the number of URLs in your Core Web Vitals report, the search company wrote.
Google wrote, “You may see a change in the number of URLs in your Core Web Vitals report. This is because more URLs are now being reported on due to a new origin group that contains data for URLs that previously fell below the data threshold. “
You should see a little marker notifying you of this update in the report, here is what it looks like:
Here is more about URL Groups from the Google help document:
URLs in the report are grouped into pages that have a similar user experience. The LCP, FID, and CLS status applies to the entire group. Some outlier URLs might have better or worse values on some visits, but 75% of visits to all URLs in the group experienced the group status shown. It is assumed that these groups have a common framework and the reasons for any poor behavior of the group will likely be caused by the same underlying reasons.
In order to respect user privacy, a URL group must have a minimum amount of data to be shown in the report. If a URL group doesn’t have enough information to display in the report, Search Console creates a higher-level origin group that should contain enough URLs and data to show in the report. This origin group contains data for all URLs in the same protocol://host:port group. For example, if the URL https://m.example.com/a/b/c.html is part of a group that doesn’t have enough data to show, then Search Console will create the origin group https:// m.example.com. This origin group contains data for all URLs under https://m.example.com, whether or not the URL also belongs to a group with sufficient data.
Note, I don’t think Core Web Vitals are that vital to SEO, so I wouldn’t get bogged down in these reports for SEO or ranking reasons.
Forum discussion at Twitter.