Honestly, this is a really common one. The root of the problem is the canonical itself. When /blog/page/3/ points its canonical back to /blog/, Google sees page 3 as a duplicate of page 1 and basically stops bothering to index anything that only lives on deeper paginated pages. That fits perfectly with what you're seeing - older posts dropping out of the index.
Since around 2019, when Google deprecated rel=next/prev, the official guidance has been pretty clear: each paginated page should have a self-referencing canonical. So /blog/page/3/ should canonicalise to /blog/page/3/, not to /blog/.
In Yoast, you can fix this under SEO > Settings > Advanced (or Search Appearance > Content Types > the relevant post type in older versions). Look for the "paginated archive canonical" setting and switch it from "first page" to "self".
A couple of other bits worth checking while you're in there:
That ?et_blog parameter at the end of your URLs - if it's a Divi session param, you want the canonical to strip it. Yoast usually does this automatically, but double-check with URL Inspection to be sure.
The /ru/ language prefix in your example. If you're running multilingual pagination with WPML or Polylang, the canonical pattern needs to respect that language prefix. /ru/blog/page/3/ should self-canonical, not point to /ru/blog/ or, worse, just /blog/.
To verify, grab one of those older posts that isn't indexing and run it through GSC URL Inspection. If it says "Alternate page with proper canonical tag" and shows /blog/ or /ru/blog/ as the canonical, that's your smoking gun.
After you make the change, resubmit the paginated URLs via URL Inspection > Request Indexing to nudge Google into re-crawling with the new canonicals. Should sort it.