I tested your code with asciidoctor-maven-plugin and it worked fine for me. I only configured the 'html5' backned.
I realize that using the maven plugin may not be a solution for you, but can be starting point to see if this is an issue with asciidoctorj.
To keep working on it, can you provide more information? For instance, can you confirm you're using asciidoctorj, which version, are you setting some other parameters, etc.
Here is my maven config:
<plugin>
<groupId>org.asciidoctor</groupId>
<artifactId>asciidoctor-maven-plugin</artifactId>
<version>1.5.2</version>
<executions>
<execution>
<id>output-html</id>
<phase>generate-resources</phase>
<goals>
<goal>process-asciidoc</goal>
</goals>
<configuration>
<backend>html5</backend>
</configuration>
</execution>
</executions>
</plugin>
PS: how do you create those nice code blocks in the forum? :O