Post Thumbnails Randomly Not Showing up

Okay so here’s the problem:

Many post thumbnails don’t show up on my blog at https://igeeksblog.com. This happens randomly: some pages render the post thumbnails correctly while some don’t. It happens in all the pages: home-page, categories, author, archive, tag etc.

For instance:
1. No Thumbnails on this category page: https://www.igeeksblog.com/category/iphone/
2. But a different page number under the same category shows thumbs: https://www.igeeksblog.com/category/iphone/page/7/

It’s not a file/media issue because thumbs for the post do show up on some other category page.

For example:
1. https://www.igeeksblog.com/category/iphone/ – Look at the first post. [iPhone 6 (4.7″)…] .. no thumbs.
2. The same post on this page: https://www.igeeksblog.com/category/news/ shows thumbnails.

Here’s what we’ve tried so far to fix the issue:
(basics: cleared all server/browser/wp cache; we haven’t edited any file, not changed/updated plugins…. nothing actually)

– bugged the theme developer half a dozen times. the guy was really helpful but it’s not the theme’s fault at all.
– changed the theme. no luck.
– disabled all plugins. no luck.
– checked, double checked, triple checked all the settings and configurations. nothing out of the ordinary. nothing changed.
– completley disabled CDN. nope, no change.
– checked wp-content/uploads manually to see if the files exist. they do. chmod 755, then 757, then 777. nothing worked.
– checked the DB thru phpMyAdmin. everything seems normal. no changes there.

there’s no more hair to pull out in trying to fix this. any help would be appreciated… please feel free to throw any kind of a suggestion.

we created a new category+slug and added posts to test it out. same problem. so we’re actually trying anything and everything – even if it seems totally absurd.