@Aarkon well java 11 is the current version, isn't it? but my issue is actually a bug in openjdk 11+ https://bugs.openjdk.java.net/browse/JDK-8212233
Their workaround is to pollute the pom to use java-8 for javadoc (and java-11 for the rest). But... no. :angry_trump:
My workaround is to replace the javadoc binary in java-11 with a link to the binary in java-8:
/usr/lib/jvm/java-11-openjdk-amd64/bin/javadoc -> /usr/lib/jvm/java-8-openjdk-amd64/bin/javado
and wait until their official fix drips down to my setup...