[RH2019] %2F in URL without #t= causes hyperlink issues

AFAIK there's only 2 ways to launch your content - (1) invoke the index.htm page - this would be the full help (with TOC visible) and you'd end up on a default topic as the landing page; (2) invoke the specific topic within the content (this is traditionally called CSH (Context Sensitive Help) because you "know" the context of where you are in the launching application and only want content about that screen presented). This CSH launch traditionally doesn't present the TOC (but offers a link to expose it and show it in its position within the TOC). Some versions of RH have monkeyed with this content-only view because of the impact on Google searches, but I believe that has been fixed now.

I have no idea how you are launching what you call an "out-of-context" page - perhaps you can describe that scenario?

I am not a specialist here, but I have had an issue with thumbnails that could not be found due to a corrupt path. This has been fixed in version 2020 update 4. 

In my case, the problem only occured when the help was embedded within an iFrame. 

For reference, you could see this original post: https://community.adobe.com/t5/robohelp/rh2019-new-ui-problem-with-thumbnails-in-html5-help/m-p/11605110#M134439

Maybe this helps you or your developer to find out what is causing the issue.

Because context sensitive help is in play I'd be looking at how the help is called from the application. That seems the most likely place for the path to be escaped like you describe. You'll need to speak to your developers to help.

I'm unclear on your terminology - what do you mean by "in context", "without context" and "out-of-context"?

Are you using RH2019 Classic or New UI? Do you have all the patches installed (last one was 14)? Which output are you generating? Is context sensitive help involved?