I have filed a bug report, too, (https://bugs.kde.org/show_bug.cgi?id=404106) so please let me know if this post is annoying .

I have used okular for a long time on ubuntu with KDE. One very important use is
part of LaTeX. For documents, everything has been just great. Until recently, it was
also great for slides, using beamer. I was able to embed videos successfully using
the multimedia package, and have them play during presentations. But since upgrading
to Ubuntu 18.04, this ability has disappeared. The okular version in the repos is 1.3.3,
and it has some limited ability outlined in the bug report. I also tried the snap package
version, but that was even worse, since it crashed okular.

My questions here: has anyone else experienced this? Has anyone found a solution?
Do I need to go back to an older version for this feature to work? There are more
details about my setup in the above-linked bug report.