Related posts with thumbs

Temporarily disable WP Rocket ( or just its Page Caching ).
Then check the site in an Incognito/Private Browser ( it has no caching ), try a few refreshes across the different posts.
This will identify, or eliminate, page caching as the issue.

So here is what I found.

I am using the shortcode in a widget to show related posts in the sidebars.

The problem is on pages, it would display "No posts found", because it was a page with no category, I think.

So I am using Custom Sidebars and created a sidebar just for pages, but anytime I added the shortcode to the pages sidebar, it would change the shortcode in the regular sidebar.

I solved the problem by using an html widget for the shortcode on the regular sidebar and a text widget for the pages sidebar.

It is currently working.

Hope that all makes sense.

Ah ok that makes sense.
Glad to hear you found a solution!