The yellow marker disappears just after I have swept the cursor over the text to highlight. However, the file is marked as having been edited and needing to be saved. After saving no marker shows up. Ditto if I close and reopen the file. So mark-up may well exist but is not visible.

Same story for lines, other annotations, and inline and pop-up notes. The various stamps work fine though.

I run Ubuntu 20.04 LTS that ships Okular 1.9.3 out of the apt standard repository.
Additional libraries according to Okular's own help menu:
[*] KDE Frameworks 5.68.0
[*] Qt 5.12.8, built against 5.12.8
[*] the xcb windowing system.
Ubuntu calls the version of this packaging of Okular 4:19.12.3-2ubuntu1

I started Okular from the command line and saw whether it threw error messages. I spotted none that I myself could link to this issue.

Out of intuition I installed a few more packages serving or extending Okular:
[*] okular-dev
[*] okular-mobile
[*] libpoppler-{cpp-dev|glib-dev|-private-dev} 0.86.1
but these did not help either

How could I possibly fix this? In Ubuntu 16, Okular 0.24.2 (KDE Development Platform 4.14.16, poppler 0.41.0) was working as expected. Text highlighting and annotations were a valuable assistance to reading, one for which I had been delaying my distribution upgrade.

Official package information for working and not working distributions are available from
[*] working, Ubuntu 16: https://packages.ubuntu.com/xenial/okular
[*] not working, Ubuntu 20: https://packages.ubuntu.com/focal/okular
I can see that the dependencies of Okular have greatly changed between the two versions.

For completeness: this post is pretty much the same as an older one of mine in AskUbuntu https://askubuntu.com/q/1212746/446253. The problem was already there with Okular 1.3.3 in Ubuntu 18.04 LTS

I also made a few experiment across two distributions if Ubuntu 16 (fully working) and of 20 (failing):
[*] I scribbled a PDF document with Okular Ubuntu 20 and viewed it with the Okular of Ubuntu 16. In Ubuntu 16 I can just see the same annotations that I could see in Ubuntu 20. Apparently, what did not persist when viewing in Ubuntu 20 has not actually been saved in the document.
[*] Conversely, I scribbled a PDF document with Okular of Ubuntu 16 and viewed it with the Okular of Ubuntu 20. In Ubuntu 20 I can see the annotations that I could see in Ubuntu 16. So these do persist.
[*] Finally, the annotations that persisted in Okular of Ubuntu 20 and could be viewed by Okular of Ubuntu 16, disappear if I view the document in Ubuntu 20 once again. That is, what I could add and view in Ubuntu 20 is not visible in Ubuntu 20 any longer if I saved that document in Ubuntu 16 in the meantime. That is, I only see the annotations added with Ubuntu 16 and nothing at all of those added by Ubuntu 20.