You'll see that people are not very talkative :-)
I'm not familiar with Fedora, but if I understand correctly https://bugzilla.redhat.com/show_bug.cgi?id=1909865
it means that PlantUML does need a complete JVM to run (and not a "java-headless" one, which seems to be a Fedora notion).
So probably you have to change some dependency list somewhere (but I have to idea where :-) ).
About OpenJDK, PlantUML should run on any JDK. So you might pickup another one. Once again, this is not on raw PlantUML side, but probably on the Fedora package itself.
Hope this helps! Tell us about your progress!