Google’s John Mueller said that if you are seeing “seeing a drop in valid, and not a rise in invalid, that’s possibly just a matter of the sample used for testing becoming smaller for that site.” This is specific to the Search Console structured data enhancement report sections.

So John is saying that sometimes what Google is looking at, the sample URLs on your site, may have changed on Google’s end. And if that happens, you may see a drop in valid or other metrics but as long as you do not see a rise in the number of invalid URLs, it may be okay. He said “that’s fine”

One way to double check, John said, would be to “test some of the URLs that are no longer listed.”

Here are those tweets:

Forum discussion at Twitter.