Their workaround is to pollute the pom to use java-8 for javadoc (and java-11 for the rest). But... no.
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...