The number of structured data items on our site fell from 4M to ~2M recently.
The parts of the site responsible for this data weren't changed in a long time and the general number of crawled/indexed pages didn't change either.
When testing the pages with Google Structure Data Testing, everything is OK, also, in Google Search Console I see that all items are OK: "0 Items with Errors"
Explanations I've found in the Internet, aren't really helpful - in most cases the problem is caused by errors in site's markup, which shouldn't be the case as we passed Google's tests, or in Google's servers error - but the last news about that are from 3 years ago, and everything was fixed in a week.
In our case it's been a month since the numbers fell.
What could other reasons for this problem be? Any ideas are welcome